The correct answer is (a).
The irreversibility rate is the difference between the heat input and the work output of the heat engine. In this case, the heat input is 400 kW and the work output is 200 kW, so the irreversibility rate is 400 kW – 200 kW = 200 kW.
Option (b) is incorrect because it is the work output of the heat engine. Option (c) is incorrect because it is the heat input to the heat engine. Option (d) is incorrect because it is the sum of the heat input and the work output of the heat engine.