The iris is a ring of muscle tissue that forms the colored part of the eye. Additionally, the iris controls the size of the pupil opening, and thus controls how much light enters the eye.

The stages of cognitive development, according to Jean Piaget, are the sensorimotor, preoperational, concrete operational, and formal operational. During the sensorimotor stage, the period between birth and age two, cognitive development is limited to motor responses caused by sensory stimuli.

Between ages two and six—the preoperational stage—the child learns to use language. Between ages seven and eleven—the concrete operational stage—children begin thinking logically. During the formal operational stage, age twelve through adulthood, people develop the ability to think about abstract concepts such as deductive reasoning. 

The middle ear contains three tiny bones: the malleus (hammer), the incus (anvil), and the stapes (stirrup). These bones concentrate the vibrations coming from the eardrum, and send these signals to the cochlea's oval window.

Each region of the ear uses a different medium to transmit vibrations. Remember that the outer ear uses air as the medium (sound waves impact the ear drum). The middle ear uses bones to transmit these vibrations to the oval window. Finally, the inner ear uses fluid found in the cochlea to transmit the vibrations to neural tissue.

The retina contains the rods and the cones, which are sensitive to light that enters through the pupil. Rods are able to detect shades of dark and light, while cones are able to detect color. When light strikes these cells, it is converted into electrical neural impulses that are transmitted through the optic nerve to the brain.

Serotonin is the neurotransmitter that is most responsible for mood control. Therefore, a lack of serotonin would likely result in clinical depression, a mood disorder.

Olfactory cells are receptors involved with our sense of smell, while the other answer choices are all related to vision, and are located in the eye. 

The trichromatic theory is one of the oldest theories about why and how humans see in color and it states that there are three types of cones in the retina that can detect the three colors (hence trichromatic) of blue, green, and red. 

As for the other answers, dichromatic theory and trichronological theory don't exist; they just sound similar to the correct answer. Place theory and frequency theory are theories about audition rather than vision. Place theory states that ear hairs respond to different frequencies based on their different locations in the cochlea, and frequency theory states that pitch is perceived by the rate at which hair cells fire. 

The area devoted to a particular region of the body in the sensory cortex is proportional to the region's sensitivity. Lips are known to contain more sensory receptors than any of the other answers. Thus making "lips" the correct answer.

It is true that visual stimulation is different from olfactory stimulation; however, the heart of the question lies in the matter of sensory receptors. It is not a matter of the receptors being different strengths, but of how they respond to a stimulus. Olfactory receptors undergo sensory adaptation. This occurs after continuous exposure to a stimulus when nerve cells will fire less. As a result, we become less aware of the smell. Visual receptors cannot undergo adaptation because our eyes are always moving. As a result, the stimulation is constantly changing for our eyes' receptors. Psychologists have devised an experiment to test sensory adaptation with visual receptors. By maintaining a constant image on the inner surface of the eye, the individual will eventually cease to see the image that was once there. The image will occasionally reappear and disappear again; therefore, it is possible to become sensory adapted visually—but not under normal circumstances. 

This phenomenon is known as sensory adaptation, where sensitivity diminishes over time with a continuous stimulus. Over time, as stimulation continues, nerve cells will fire less and as a result we will no longer notice a stimulus—in this case, touch.