The Department of Transportation (D.O.T.) must build and maintain roads. Option C is correct.
Sentences with an action verb, the subject performs the action denoted by the verb, however, we may change the normal word order of many active sentences, which are those which have a direct object, so that the subject is no longer active, but is, instead, being acted upon by the verb - or passive.
The passive voice of the active sentence in this case would be:
Roads must be built and mantained by the Department of Transportation (D.O.T.).