Concept:The verb "prefer" always takes the preposition "to" when comparing two choices.
Explanation:The original sentence uses "rather than" after "preferred".
This is incorrect because "prefer" is never followed by "than".
The correct structure is: prefer X to Y.
Here, death is compared with imprisonment.
So the correct preposition is "to".
For example: "I prefer tea to coffee."
The other options, "for" and "than", are not used with "prefer".
Thus the improved sentence is: "He preferred death to imprisonment."
Answer:Option B – "to"