Choose the word that correctly completes the analogy. common: scarce : : human-made: a. chalk b. cavern c. process d. natural

The correct answer would be D.natural. As you may have infered, both pairs of words are opposites or are the antonym of each other. The term human-made is a synonim to "man-made" which is defined bythe  Oxford dictionary as "Made or caused by human beings (as opposed to occurring or being made naturally)" and natural, when used as an adjective has the following meaning "Existing in or derived from nature; not made or caused by humankind." it is safe to state that the only coherent answer is in fact letter D.

When analyzing analogies such as the one presented before one must consider the pattern that reveals itself in the first pair of words. They could be synonims, antonyms, adjectives, pronouns, etc. The important thing to remember is to take into consideration every aspect of the words in order to find the pattern, for example the construction, the meaning, the use, etc.

what did the protagonist say?