The correct answer is A) Piston rod and crosshead construction. Piston rod and crosshead construction is the most common design feature of a two-stroke cycle, low-speed main propulsion diesel engine. This design provides better support for the piston assembly and allows for a longer stroke, which is characteristic of low-speed diesel engines. The crosshead design also helps to separate the crankcase from the cylinder, preventing lubricating oil from entering the combustion chamber. The other options are not as closely associated with two-stroke, low-speed diesel engines. The 90-degree 'V' configuration (B) is more common in higher-speed four-stroke diesel engines. Double acting trunk type pistons (C) are less common, and single reduction gearing (D) is a general feature found across various engine types, not specifically two-stroke low-speed diesels.