Portable means movable or mobile.

"Periodical" and "occasional" are synonyms describing periods of time. The other words do not have meanings that relate to time. 

"Momentary" and "passing" are synonymous adjectives similar in meaning to the word fleeting or temporary. The other words do not have related meanings. "Eternal" and "timeless" are synonyms describing the property of being immutable.

"Elderly" and "senior" are both closely related in meaning. These answer choices, however, are not perfectly aligned with "anachronistic," which is in fact synonymous with from a time other than the present time. "Soon" and "recent" are also closely related words related to time, they are more specific terms than anachronistic.

The adjective "provisional" means temporary, conditional, designed to be impermanent. Additionally, "permanent" means lasting forever; "elusive" means hard to catch, hard to find, hard to identify; "erroneous" means based on a mistaken belief; "aloof" means distant and withdrawn, detached from human contact and unfriendly.

The adjective "contiguous" means not separate, touching one another, adjacent, or adjoining. Additionally, "beatific" means joyous and blissful; "incendiary" means flammable or likely to cause an offended and passionate reaction; "capricious" means fickle, inconsistent, and changeable; and "abominable" means loathsome and hated.

The adjective "inexorable" means unstoppable, "relentless," impossible to prevent from happening. Additionally, "unscathed" means unharmed by; "indispensable" means vital, needed, essential; "abstruse" means (of knowledge) hard to understand, esoteric, arcane, recondite; "poised" means showing calm and composure in a stressful situation.

The adjective "precipitous" means dangerously steep, sheer. It can also mean sudden and abrupt. Additionally, "dormant" means sleeping, at rest, not active.

The adjective "inopportune" means occurring at the wrong time, inappropriate for the moment, or "untimely." To provide further help, "untimely" means happening at a time deemed unfortunate or inappropriate. Additionally, "enigmatic "means mysterious, puzzling; "pusillanimous" means cowardly, not brave; "assiduous" means hard-working, meticulous, showing 'attention to detail'; "insidious" means treacherous, advancing in a deceptively slow fashion that nonetheless leads to devastating consequences.

The adjective "episodic" means occurring in episodes, occurring in a series of regularly repeating segments. So, a television show could often be described as "episodic." This definition is closest in meaning to "intermittent" which means occurring at irregular intervals, not continuous. Additionally, "fallow" means (of soil) not being used to grow crops; "irresolute" means uncertain, unsure; "acclaimed" means widely praised; "limpid" means (of a liquid) clear.