Answer
A
Work Step by Step
Since all compounds are trichlorides, the one with the highest mass percent of chlorine is the one where the other atom has the lowest atomic weight: $B - 10.86\ g/mol$.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.