Correct Answer:
B. For
The correct preposition to use in this sentence is 'For'. The preposition 'for' is used to indicate a duration or length of time. In the phrase 'We discussed the issue ___ an hour,' 'an hour' specifies the total period over which the discussion took place. Therefore, 'for an hour' accurately conveys this duration. 'Since' is used to indicate a starting point in time (e.g., since 9 AM). 'From' typically indicates a starting point or origin. 'During' means throughout the course of, but 'for' is more precise when stating the exact length of time something occurred.