The force responsible for balancing the force due to gravity on a metal ball suspended by a rod at rest is tension in the rod.

Here's why:

* Gravity: The Earth's gravity pulls the ball downwards, creating a force called the weight of the ball.

* Tension: The rod is holding the ball up, preventing it from falling. This upward force exerted by the rod is called tension.

Since the ball is at rest, the forces acting on it must be balanced. This means the tension in the rod is equal in magnitude but opposite in direction to the weight of the ball.