Fair value of investment in equity instrument

It will state in the question that the gains/losses arise evenly over the year unless told otherwise. If we're not told otherwise, which is likely the case here, then we pro-rate it.

But the loss on fair value of investment in equity instrument seems to be a capital loss and not revenue loss which seems to occur at a point in time like revaluation surplus and not over the year so how could it be pro-rated

Agree but just follow as instructed in the question.