The correct answer is: B. We are taught grammar by him.
The sentence “He teaches us grammar” is in the present tense. The verb “teach” takes the object “us” and the direct object “grammar.” The passive voice of the sentence is “We are taught grammar by him.” This means that the subject of the sentence is now “we” and the verb is now “are taught.” The direct object “grammar” remains the same.
The other options are incorrect because they do not agree with the tense of the original sentence. Option A, “Grammar was taught to us by him,” is in the past tense. Option C, “Grammar will be taught to us by him,” is in the future tense. Option D, “We were teached grammar by him,” is incorrect because the verb “teach” is not conjugated correctly in the past tense.
In conclusion, the correct answer is: B. We are taught grammar by him.