The correct option is A often
While framing any sentence, the basic rule is - Subject + verb + object (if any). 'They' is the subject, 'go' is the verb and 'movies' is the object. 'Often' is an adverb of frequency. The rule says that if a sentence has one verb, place the adverb of frequency in the middle of the sentence so that it is positioned after the subject but before the verb. 'Happily' is an adverb of manner thus option B is wrong. 'Do not' is not an adverb. Thus, according to the rule, option A is the correct answer.