The question said the lift accelerate upward,therefore the weight on the lift should be F=mg+na.the answer I got was C

I think the answer should be 7.5N. Did you attempt to solve it yourself? How did you go about it?

Gravity acts downwards and the lift accelerates upwards, so the resultant acceleration of the mass is downward due to gravity and equal to \(10 - 2.5\)ms\(^{-2}\) = \(7.5\)ms\(^{-2}\). 
Therefore, the reading on the spring balance which corresponds to the weight of the mass is equal to \(ma\) = \(1\)kg \(\times\) \(7.5\)ms\(^{-2}\) = \(7.5\)N.

A spring balance which is suspended from the roof of a lift carries a mass of 1kg at its free end. If the lift accelerates upward at 2.5ms\(^{-2}\) determine the reading on the spring balance [g = 10ms\(^{-2}\)]

Earlier discussion