The correct answer is A) as steam passes through the steam admission valve and there is a drop in pressure without the performance of work. Turbine throttling losses occur when steam passes through the steam admission valve (also called the throttle valve) of the turbine. As the steam flows through the valve, there is a drop in pressure without the steam performing any work. This pressure drop represents a loss of energy, which is known as the turbine throttling loss. The other answer choices are incorrect because they do not accurately describe the nature of turbine throttling losses. Option B refers to steam leakage, option C describes friction in the nozzle block, and option D discusses friction caused by frequently throttling the turbine wheel and blade speed, which are not the primary causes of turbine throttling losses.