Question

When an adverb modifies a verb with a helping verb, the adverb should generally be placed A. at the beginning of the sentence. B. at the end of the sentence. C. before the helping verb. D. after the helping verb.

Answer 1

You can place the adverb between the helping verb and the main verb. I presume by "helping verb" you mean something like a modal auxiliary or a regular auxiliary?

Example. You should never smoke.
("never" is a frequency adverb)

We must quickly finish this assignment

But you could also say:
We must finish this assignment quickly

He is forever telling bad jokes.

The man was quietly talking to the nurse.

But you can also say,
The man was talking quietly to the nurse.

So while there is no strict rule that says the adverb is placed between the auxiliary and main verb, it can happen.
However, frequency adverbs (always, never, usually, sometimes) are almost always placed between.