The correct answer is D. raining.
The sentence “It had been raining since morning” is in the past progressive tense. The past progressive tense is used to describe an action that was happening at a specific time in the past. In this case, the action is “raining.” The past progressive tense is formed with the past tense of the verb “to be” (was or were) and the -ing form of the main verb (raining).
A. rain is the infinitive form of the verb “to rain.” It is not the correct answer because it is not a verb in the past progressive tense.
B. rained is the past tense of the verb “to rain.” It is not the correct answer because it does not describe an action that was happening at a specific time in the past.
C. Both A and B is not the correct answer because only one of the options is correct.
Therefore, the correct answer is D. raining.