The correct answer is: less.
An abnormal gain is a gain that is not expected. When actual loss is less than the estimated loss, the difference between the two is considered to be an abnormal gain. This is because the company has actually made more money than it expected to.
Option A is incorrect because it says that actual loss is more than the estimated loss. This would result in an abnormal loss, not an abnormal gain.
Option B is incorrect because it says that actual loss is equal to the estimated loss. This would not result in an abnormal gain or loss.
Option C is incorrect because it says that actual loss is higher than the estimated loss. This would result in an abnormal loss, not an abnormal gain.