What do you mean by fractional column?

A tall, horizontally subdivided or packed container for fractional distillation in which vapor passes upward and condensing liquid flows downward. The vapor becomes progressively enriched in more volatile components as it ascends, and the less volatile components become concentrated in the descending liquid, which can be drawn off.

The most plausible answer is b, duration. the given amount of time is considered how long it last for.