In the original sentence, "Could you pass the test?" the subject is "you," the verb is "could pass," and the object is "the test."

When transforming the sentence into passive voice, the object becomes the subject, the verb changes to its passive form, and the original subject may be omitted or introduced with "by."

The correct passive voice form is "Could the test be passed?" Here, "the test" is the new subject, "could be passed" is the passive verb construction, and "you" is omitted in the passive transformation.