"Imminent" is defined as likely to occur at any moment, and "impending" is defined as about to happen. These similar definitions make the two words synonyms. 

The word "erratic" means constantly changing or having no certain state. "Irregular" is the best choice because it has a related definition—not regular. Considering the other choices, "true" means accurately reflecting things as they are or correct, "rational" means agreeable to reason, "tacit" means understood without being said, and "flagrant" means brazen or done without care for the obviousness with which one is breaking rules.

"Juvenile” means immature and behaving like a child or young in age, so "childish" is the correct answer. "Variegated" means containing many different colors, especially in a stripe-like pattern; "portentous" means ominous; and "wily" means clever and sneaky.

“Auspicious” means favorable or lucky, so "favorable" is the correct answer. As for the other answer choices, “exuberant” means excited and enthusiastic; “permanent” means lasting or unable to be erased; “transitory” means temporary or not permanent; and “orthodox” means conventional or abiding by a given set of rules.

“Stationary” means unmoving as opposed to in motion, so "unmoving" is the correct answer. As for the other answer choices, “staunch” means committed and firmly supporting; “frosty” means cold; “stern” means strict and sticking to the rules; and "fluid," when used as an adjective, means either flowing or in a liquid form.

The word "sporadic" means happening at irregular intervals in time. The best choice is "intermittent" because it shares the same definition. Considering the other choices, "arboreal" means having to do with trees, "spartan" means self-disciplined and apathetic about comfort, and "burdensome" means troublesome.

"Permanent" means lasting forever or for a very long time or not erasable. To provide further guidance, "insipid" means boring or dull, "optimal" means perfect or prime, and "coherent" means able to be understood.

Transient most nearly means temporary. Something that is "transient" is passing or fleeting. As a noun, the word most nearly refers to a vagabond. For example, "her feelings were transient."

The other choices are antonyms, with the exception of "peaceful," which is unrelated.

"Obsolete" means outdated or no longer in use, so "outdated" is the correct answer. None of the other answer choices are close in meaning to "obsolete": "modern" is an antonym of "obsolete" and means new or up-to-date; "stubborn" means refusing to change one's attitude or viewpoint despite evidence to the contrary; "pious" means devout or very religious; and "wretched" means miserable or very sad.

Something that is "obsolete" is no longer useful or out of date, so "out of date" is the correct answer. To help you, "fecund" means fertile, "demonstrative" means openly showing how one feels, especially concerning being fondness, and "prolific" means productive, creative, or fecund.