Final answer:
The forces acting on a baseball thrown into the air are gravity, causing it to fall, and air resistance, which opposes its motion. Hence, the correct answer is option (a).
Step-by-step explanation:
When Anton and Brian throw a baseball into the air, the forces acting on it are gravity and air resistance. The force of gravity pulls the baseball toward the Earth, causing it to eventually fall. Air resistance, which is a type of drag force, opposes the motion of the baseball as it moves through the air, and the magnitude of this force will depend on factors such as the baseball's velocity and the properties of the air.
While magnetic forces are relevant in some contexts, they do not significantly impact the flight of a baseball. Hence, the correct answer is (a) Only gravity and air resistance (though that may be small).