Answer
Please see the work below.
Work Step by Step
We know that $As_2S_3$ particles are negatively charged, the effective coagulation demands a highly charged cation, hence $Al_2(SO_4)_3$ is the right option.
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.