The correct option is D a couple that has worked
There are two numbers in Noun-Number: Singular and Plural. When we speak about one person / thing, we use the noun in singular form. When we speak about more than one person / thing, we use the noun in plural form.
In the given sentence, the word 'are' indicates that the noun just after it should be a plural noun. This rules out options A, B and C. Even though option D has the collective noun 'couple' which is singular, it is the right option since it correctly identifies that the two people have worked.