The correct option is A seldom
'Seldom' 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. 'So' and 'Do not' are not adverbs. Thus, according to the rule, option B is the correct answer.