The correct answer is D) Upper and lower crankshafts. Opposed-piston engines, which are commonly used in large marine diesel applications, are the only type of engine that has both an upper and lower crankshaft. This unique configuration is what distinguishes opposed-piston engines from other engine types, such as traditional four-stroke engines which have a single crankshaft. The other answer choices (A, B, C) are all components that can be found in various engine types, not just opposed-piston engines. Therefore, the presence of upper and lower crankshafts is the defining characteristic that is unique to the opposed-piston engine design.