He teaches us grammar.

The correct answer is Option D: We are taught Grammar by him.
Let's break down why:
The original sentence "He teaches us grammar" is in the active voice.
In the active voice, the subject (He) performs the action (teaches) on the object (us, grammar).
To change it to the passive voice, we need to:
1. Identify the object that becomes the subject. In this case, "us" can become "we".
2. Use the correct form of the verb "to be" (is, are, was, were, etc.) that matches the tense of the original sentence. Since the original sentence is in the present simple tense ("teaches"), we need the present simple form of "to be," which is "are" for "we".
3. Use the past participle of the main verb ("teach"). The past participle of "teach" is "taught".
4. Include the original subject ("He") as the object of the preposition "by". This becomes "by him".
Therefore, the correct passive voice sentence is: "We are taught Grammar by him."
Let's look at why the other options are incorrect:
Option A: Grammar is taught to us by him. While grammatically correct, this focuses on "Grammar" being taught, which isn't the direct change from "He teaches *us*". It's a less direct transformation of the original sentence.
Option B: We are being taught Grammar by him. This implies a continuous action (present continuous passive), which isn't the tense of the original sentence.
Option C: Grammar is being taught us by him. This is grammatically incorrect. You can't "teach us" something directly in the passive voice like this. It should be "Grammar is being taught *to* us" if attempting a continuous passive tense. However, even with the corrected "to", option B is better because the original sentence isn't continuous.