Answer:
D - yet
Explanation:
None of the other words are conjunctions: dress is a noun, are is a verb and of is a preposition. Only yet can be used to coordinate elements (i.e., link elements at the same level) in a sentence. In this case, the elements are the adjectives simple and elegant, but yet can also coordinate whole clauses (e.g. I told you she was lying, yet you never believed me).
