She was surprised by his amiability.

Answer: Option D

Amiability : the quality of having a friendly and pleasant manner; geniality.
Pessimism : a tendency to see the worst aspect of things or believe that the worst will happen.
Prudence : the quality of being prudent; cautiousness.
Sincerity : the absence of pretence, deceit, or hypocrisy.
Petulance : the quality of being childishly sulky or bad-tempered.

Antonym of Amiability is Petulance