The correct answer is: A step cost.

This cost remains fixed within a certain range of activity, and then 'steps up' to a higher cost when the activity level increases beyond this range.

This cost is not fixed, because the cost increases as the activity level increases.

This cost is not variable, because the cost does not vary directly with the number of units produced. This is apparent from the cost being equal at two different levels of production (150 and 200 units).

This cost is not a semi-variable (mixed) cost, which is a cost that contains a fixed element and a variable element. Given that the cost is equal for two levels of production (150 and 200 units) this cost could not be made up of a fixed element together with a cost that varies directly with production.