Answer:
C. Jealousy.
Explanation:
The phrase "to drain the cup of bitterness" implies to be a matter of a character's choice, not problems that surge invoruntarily as rejection and hardships would - leaving poison or jealousy as the only options.
It is logical to become jealous out of bitterness, so it is fair to choose Jealousy as a fit comparison with the "cup of bitterness" metaphor, where as poison would be better fit for a "cup of death" - such one would expect as a consequence for drinking poison.
