The correct answer is A) Curtis stage, impulse turbine. The astern element of a main propulsion turbine is usually designed as an impulse turbine, specifically a Curtis stage. This is because impulse turbines are well-suited for the astern (reverse) operation of the main propulsion turbine. The Curtis stage, which features multiple rows of impulse blading, is a common design for the astern element as it provides an efficient and effective way to generate the required reverse thrust. The other options are incorrect because: B) is a different turbine configuration not typically used for the astern element, C) is a reaction turbine which is not the standard design for the astern element, and D) is an unusual turbine configuration not commonly used for the astern element.