Answer:

C.

Explanation:

Choosing whether to use an adjective or an adverb depends on what you want to that word to describe.

The rule is: nouns can be modified by nouns or adjectives; but adjectives, adverbs, and verbs can only be modified by adverbs.

In this case, it’s important to first figure out what function the underlined words are playing. The sentence is explaining that each feather has a different shape from those around it. That means that the verb “shaped” is being modified by the word explaining variations in form. Since it’s modifying the verb, it needs to be an adverb, so it has to be “differently.” This means answers A and D are out.

But the sentence also points out that the feathers aren’t all that different from one another – their shapes have mild, not extreme variance. So the adverb “differently” is being modified by the word expressing the degree of difference. Since it’s modifying an adverb, it has to be “slightly,” so answer C is correct.