答案：D

D　句意：意志力是一种品质，是做好任何事情所需要的。“________is________it takes to do anything well”为非限制性定语从句，先行词为willpower，先行词在定语从句中作主语，故用which引导；“________is________it takes to do anything well”这一定语从句中又含一表语从句，即“________it takes to do anything well”，该表语从句缺少宾语(作takes的宾语)，表示“事物”且语境中并无选择含义，故用what引导。