选A which
I will never forget the month which we spent together in the country
我永远不会忘记我们在农村一起度过的那个月
理由：
因为这题虽然先行词是the month 但如果选C when 那么从句就缺少宾语,spent 是及物动词,后面没宾语句子就不完整,故用that/which引导,作spent的宾语.即we spent the month together我们一起度过的那个月