Demur means to protest, raise objections or express doubts. So, its antonym is embrace, which means to accept gladly.
'Quiescent' means inactive, dormant, and sluggish. So, its antonym is active.
Sapient means wise, knowledgeable, etc. So, its anonym is simple.
'Impecunious' means needy, poor, penniless. So, its antonym is rich.
'Rueful' means regretful or remorseful. So, its antonym is joyful.