Rewrite the sentence in passive voice.
Were you washing the clothes?

Correct option is B

Active Voice: In Active voice the subject is the one doing the action.
Passive Voice: In Passive voice the subject is the recipient of the action.
To convert active voice into passive voice the object becomes the subject and the subject becomes the object.
For Simple Past Tense as given in the question the structure for changing a sentence from active to passive and vice versa is:
(Active voice) Subject + V2+ object.
(Passive voice) Object+ was/were + V3+ by + subject.