C)should, have, could
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, to have and to do. The modal ones include can, could, should, may, might, will, would, shall, etc. And they can be combined to express different ideas.
In this case, "should have" 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 "could" is used to express a past ability and possibility, and it works as an auxiliary to the main verb "help".
