Final answer:
Darrow has been given the nickname 'Reaper' in the book series 'Red Rising' by Pierce Brown.
Step-by-step explanation:
Darrow has been given the nickname 'Reaper'.
The nickname 'Reaper' is associated with Darrow in the book series 'Red Rising' by Pierce Brown. In the series, Darrow is the main character who becomes a member of a group known as the 'Reapers' due to his skill and prowess in combat.
Therefore, the correct option is a) Reaper.