Select the most appropriate antonym of the given word. Complex

Concept: An antonym is a word whose meaning is the opposite of a given word. A word can usually be opposed along more than one axis of meaning, so the method is fixed: first note the part of speech of the given word, then state its core sense in plain terms, and finally take the word that reverses that same sense. A word that reverses some other property, or that belongs to a different part of speech, is not the antonym however contrary it may feel.

Application: The given word is the adjective "complex", whose core sense is "made up of many interconnected parts, and therefore intricate or hard to analyse" — the sense used in the passage line "The development of cataract is a complex process." Its axis of meaning is intricacy: how many parts a thing has and how hard it is to grasp. Reversing that same sense gives "having few parts, plain, easy to understand or to do", which is the adjective "simple".

Cross-check: Set every offered word against its own part of speech and its own sense axis:

Among the words offered, "simple" is the one that shares the intricacy axis with "complex" and reverses it; "gradual" and "adjustable" belong to unrelated axes of meaning — the speed of a change and adaptability — so neither reverses intricacy at all, and "enhance" is a verb, so it cannot be the antonym of an adjective. The passage reads the same way: a complex process is naturally contrasted with a simple one.

Answer: simple.