Step 1: Understanding the Concept:
Physical activity is any bodily movement produced by skeletal muscles that results in energy expenditure.
Step 2: Detailed Explanation:
(A) Basketball match: This is considered a "Sport" (structured, competitive).
(B) Running a marathon: This is a classic "Physical Activity" (specifically exercise/endurance).
(C) Watching a game: This is a sedentary activity.
(D) Playing chess: This is a mental game with negligible physical movement.
Note: While (A) is also physical, in general categorization, (B) is the purest example of a physical effort/activity type.
Step 3: Final Answer:
Running a marathon (B) is a clear, high-expenditure physical activity.