The correct answer is B. Family members set places at the table for relatives who cannot attend.
The original, passive sentence was - <em>Places are set at the table by family members for relatives who cannot attend. </em>In order to revise this sample sentence into active voice, we need to put the subject of the sentence first - those who perform the action of setting the table - family members.
Therefore, options C and D are incorrect because the subject doesn't come first here - these two examples are still passive sentences. Options A and B are active; however, A is incorrect because members who aren't present cannot possibly set the table. Thus, the only correct answer is B.