What is a couplet, in the context of a Shakespearean sonnet?

a) the final two lines

b) any two lines that rhyme

c) two adjacent syllables in which the first syllable is stressed

d) two adjacent syllables in which the second syllable is stressed

I just took the test and the final answer is in fact A

The correct answer is the option a) the final two lines. In a Shakespearean sonnet, there are fourteen lines, in the three quatrains, a poet establishes a theme or a problem that he is going to resolve in the final two lines called the couplet. It's true that the couplet has the rhyme scheme but that's more a characteristic than what a couplet is, and also not any two lines will work for a couplet, for that reason the option b) is incorrect.

This is a type of lyric poetry in which the couplet is always intended, where these two lines rhyme at the end. For that reason both option c) and option d) don't have rhyme or have the last two lines, they talk about adjacent syllables with first and second syllables stressed. That's not a couplet that's stress and meter in poetry. For that reason both c) and d) are incorrect. If we were talking about the meter in a Shakespearean sonnet we will find that he wrote his sonnet in Iambic pentameter, that means that it has 10 syllables with an unstressed syllable and a stressed syllable that is usually the second one.