Final answer:
The correct answer is option a. Cassandra was condemned to have the gift of prophecy but never be believed as a punishment for rejecting Apollo.
Step-by-step explanation:
For rejecting Apollo after promising to love him, Cassandra was condemned to have the gift of prophecy but never be believed. This punishment was given to her by Apollo as a result of her betrayal.
Cassandra was able to foretell the future, but due to the curse, no one would listen or believe her predictions. This tragic fate led to many unfortunate events in Cassandra's life and ultimately contributed to her downfall.