An action in a sentence can be represented in two ways, namely active and passive voice.
The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in the active voice.
The correct answer is B) Students are taught by the professor.
Option B is correct as the subject of the given sentence ( the professor) comes in the place of the object (students) and vice versa and the transformed sentence is also in the simple present tense and is transformed using the past participle form of the main verb with the helping verb (are-as 'students' is plural').
Options A, B, and D are wrong as the given sentence is not in the present continuous tense, the subject of the given sentence is not used in the place of the object, and the helping verb 'are' is missing respectively.