Question

We collected shells. We wanted to use them to make a necklace.

Answer 1

D. We collected shells because we wanted to use them to make a necklace.

Answer 2

The answer to your question would be that the sentence that joins the two sentences correctly by adding a subordinate clause is the following one: We collected shells because we wanted to use them to make a necklace. That is, the correct option would be D.

In this case, the subordinate clause introduced by the subordinating conjunction "because" introduces the reason why they collected the shells.