Answer:
D. Ellie, Michael, and he must design the table before they can build it.
Explanation:
He is the correct pronoun to be used in subject position -see italics above-. He is a personal pronoun, so it can be substituting for a male person whose function is of agent. This means that he / male person will do something : to design the table. The same applies to they - see italics above-. They is a personal pronoun substituting for Ellie, Michael and he ,and they also has the function of agent; they will build the table.