Submit the forms ____ you are finished.

Answer: A. when

The correct answer is A. when.
"When" is a conjunction used to indicate the time that something happens.
It connects the action (submitting forms) to the completion of the task.
"But" (B) is used for contrast, which doesn't fit here.
"Whether" (C) and "Or" (D) are used for choices or possibilities.

Answering after opening this box earns no Practice Points.