Question

—_______ do you surf the Internet ?
—Once a month

1. A.
   How soon
2. B.
   How old
3. C.
   How often
4. D.
   How long

Answer 1

C
根据Once a month，可知是询问频率。How soon是多久之后会发生某事；How old问年龄多大；How often 多久一次，回答频率；How long多长。所以选C。