Answer: d) When my children go to visit my mother, they have fun.
Explanation:
An independent clause contains a subject and a predicate. The subject of d) is <em>they</em> and the predicate is <em>have fun</em>. It is the only option that has both a subject and predicate (ie. is a complete sentence). Also of note is that in d) <em>When my children go to visit my mother</em> is a dependent clause.