When compared to the leg muscles of an olympic sprinter, the muscles of an olympic marathoner would likely show a greater proportion of oxidative slow-twitch fibers?

This is a true statement.

The human body has two types of skeletal muscle fibers: slow-twitch and fast-twitch. Slow-twitch fibers are used for long-endurance actions, such as a marathon, while fast-twitch fibers are used for sudden bursts of movements such as a sprint. Therefore, an Olympic marathoner will have a greater proportion of oxidative slow-twitch fibers when compared to an Olympic sprinter.