At last the bus came. I _______ for 20 minutes.

The correct option "B. Had been waiting" follows the past perfect continuous tense rule. 

In English, we use the past perfect continuous tense to talk about actions that were ongoing in the past, leading up to another action or point in time. In this case, the action of waiting for the bus (waiting) started before another past action (the bus coming). 

So, "I had been waiting for 20 minutes" indicates that the action of waiting started in the past and continued up to the point when the bus finally came. This tense is formed using the past perfect form of "to have" (had) plus "been" (the past participle of "to be") and the present participle of the main verb (waiting).