The correct option is B Was
The context refers to an action completed in the past. It requires a past tense form of the base verb 'be'.
"Is" in option A is incorrect as "is" is used to indicate the simple present.
"Was" in option B is correct as "was" is used to indicate the simple past tense. It correctly indicates the weather yesterday (past).
"Were" in option C is incorrect as it is used to indicate the simple past of 'be'. However, we need the simple past of the auxiliary verb 'is'.
"Wasn't" in option D is incorrect as the answer to the interrogative statement begins with 'yes'. Thus, it indicates agreement over the question. So, the weather was hot.
Therefore, option B is the correct answer.