The correct answer is C. Less than empty operating weight.
Zero fuel weight is the weight of an aircraft with no fuel on board. It includes the weight of the aircraft structure, engines, landing gear, and all other equipment. Empty operating weight is the weight of the aircraft with all fluids and crew on board, but no fuel. Therefore, zero fuel weight is always less than empty operating weight.
Option A is incorrect because zero fuel weight is not equal to empty operating weight. Option B is incorrect because zero fuel weight is not equal to maximum landing weight. Option D is incorrect because zero fuel weight is not equal to the sum of empty operating weight and the maximum payload.