Answer:
How often does Mr. Thapa visit his sister?
Step-by-step explanation:
Changing a sentence/ statement into a "how often" question means that the sentence will be a question and must be correctly punctuated with a question mark. Also, the question will start with the "how often" phrase and pose a question that the time phrase will answer.
This means that the time phrase "once a month" is the answer. Therefore, the question will be framed according to the answer. Also, the tense of the given sentence is simple present tense. So, the question form will also have the present tense.
Thus, the correct answer will be
How often does Mr. Thapa visit his sister?