The correct answer is C. Was.
The verb “to be” has different forms depending on the tense and the subject of the sentence. In this case, the subject is “I” and the tense is simple past. The simple past form of “to be” is “was” for singular subjects and “were” for plural subjects.
In this sentence, the speaker is talking about a time in the past when they were able to play chess. The speaker is not talking about a current ability, so the simple past tense is used. The speaker is also talking about themselves, so the singular form “was” is used.
The other options are incorrect because they are not the correct tense or form of the verb “to be.” Option A, “am,” is the present tense form of the verb “to be.” Option B, “were,” is the plural form of the simple past tense of the verb “to be.” Option D, “is,” is the present tense form of the verb “to be” for plural subjects.