"Burgeon" is a verb that means "beginning to grow or increase rapidly, flourishing," or "putting forth young shoots; budding" when used of plants, so we need to pick out another adjective that means something like "flourishing." Since "escalating" means "increase rapidly" or "become or cause to become more intense or serious," "escalating" is the answer choice closest in meaning to "burgeoning," so it is the correct answer.

“Engrossed” is an adjective that means “having all one’s attention or interest absorbed by someone or something.” So, we need to pick out an answer choice that means something like “absorbed.” That means that “bored” cannot be the correct answer, because “bored” is an adjective that means “feeling weary because one is unoccupied or lacks interest in one’s current activity,” making it an antonym, not a synonym, of “engrossed.” “Riveted,” however, is an adjective that can mean “attracted and completely engrossed,” and because “riveted” is the answer choice closest in meaning to “engrossed,” “riveted” is the correct answer.

“Null” is a word that can be used as an adjective, noun, or verb, but because all of the potential answer choices are adjectives, we will need to consider “null” as an adjective. When used as an adjective, “null” means either “having no legal or binding force; invalid” or “having or associated with the value zero.” So, we need to pick out an answer choice that means something like “cancelled” or “associated with the value zero.” That means that “valid” cannot be the correct answer, because “valid” can mean “legally binding due to having been executed in compliance with the law” or “legally or officially acceptable,” making it an antonym, not a synonym, of “null.” “Void,” however, when used as an adjective, means “not valid or legally binding,” and because “void” is the answer choice closest in meaning to “null,” “void” is the correct answer.

“Ingrained” is an adjective that can mean either “firmly fixed or established; difficult to change” when referring to a habit, belief, or attitude, or “deeply embedded and thus difficult to remove” when referring to dirt or a stain. So, we need to pick out an answer choice that means something like “difficult to change” or “deeply embedded.” That means that “transient” cannot be the correct answer, because “transient” means “lasting only for a short time; impermanent,” making it an antonym, not a synonym, of “ingrained.” “Entrenched,” however, is an adjective that is used of an attitude, habit, or belief and means “firmly established and difficult or unlikely to change; ingrained.” Because “entrenched” is the answer choice closest in meaning to “ingrained,” “entrenched” is the correct answer.

“Insidious” is an adjective that can mean either “proceeding subtly, but with harmful effects” or “treacherous and crafty,” so we need to pick out an answer choice that means something like “subtly harmful” or “treacherous.” That means that “overt” cannot be the correct answer, because “overt” means “done or shown openly; plainly or readily apparent, not secret or hidden,” making it an antonym, not a synonym, of “insidious.” “Pernicious,” however, is an adjective that means “having a harmful effect, especially in a gradual or subtle way,” and because “pernicious” is the answer choice closest in meaning to “insidious,” “pernicious” is the correct answer.

"Contaminated" means polluted.

"Toxic" means harmful or poisonous.

"Deliberate" means intentional; Accidental means unintentional; Harmful means causing harm; Treacherous means dangerous; and Illegal means not lawful.

"Prosperous" means financially successful or flourishing.

"Infectious" means contagious.