The correct answer is: A. did/find
The sentence is in the past tense, so the verb “find” should also be in the past tense. The options “do/find” and “were/found” are in the present tense and past participle tense, respectively, which are not consistent with the tense of the sentence. The option “did/found” is in the past tense and past participle tense, which is consistent with the tense of the sentence.
The sentence can be paraphrased as:
A: “I’ll drink tea from this tin mug.”
B: “Where did you find it?”
In this paraphrase, the verb “find” is in the past tense because it is describing an event that happened in the past. The option “did/find” is the most appropriate option because it is in the past tense and consistent with the tense of the sentence.