The correct option is A since
The clause "they moved to California" is subordinate as it depends on the sentence "The Smiths have had two children" for its full meaning. Hence a subordinating conjunction is used. Also, the events "had two children" and "moved to California" happen one after the other, so they're time-related. 'Though' is used when the clauses are contrast in meaning, as they're not contrast here, the underlined word needs to be replaced by 'since' which is a time-related conjunction, hence Option A is correct and D is incorrect. 'Despite' is used to show a contrast between two sentences. Since there is no contrast in meaning of the sentences here, one can have children if they move to California, there isn't any opposite meaning here, hence option B is incorrect. 'Although' and 'though' are similar in meaning, as it's a conjunction used for contrast sentences, option C is also incorrect.