a. sang
You sang well at the contest last year. The simple past tense of sing is sang therefore it should be the right answer for this question. On the other hand, sung is the past participle of the word sing that can be used in another way other than the simple past tense. The past tense of the verb was determined due to the clue "last year" which means that the action verb already happened in the past. The verb "sing" is an example of an irregular verb. Its past tense is not formed by adding 'd' or 'ed' unlike the regular verbs which take the past tense by adding 'd' or 'ed' to the verb.