<u>C)should, have, could</u>
The purpose of helping (or auxiliary) verbs is to help the main verb in a sentence to express a more complete an accurate idea. They can express possibility, probability, necessity, expectation, obligation, potential, or directions, and also can provide details to correctly express a verbal tense.
There are two types, the primary helping verbs, and the modal helping verbs. The primary helping verbs are to be, <u>to have</u> and to do. The modal ones include can, <u>could</u>, <u>should</u>, may, might, will, would, shall, etc. And they can be combined to express different ideas.
In this case, <u>"should have"</u> works as an auxiliary verb to the main verb "donated". Both verbs are combined to express a present wish, idea or obligation about a past event or a past decision, that is to say, to express something that should have happened in the past but it did not happen.
And <u>"could"</u> is used to express a past ability and possibility, and it works as an auxiliary to the main verb "help".