Strong muscles are necessary to reinforce the that hold the bones together.

The right answer is Joints (or ligaments more precisely).

Muscle is the contractile structure that produces strength. It has components that allow it to be shortened to create what is called a muscle contraction.

The joint is junction of two parts of the skeleton allowing them relative mobility.

The ligament is an elastic structure that stabilizes the joints. It therefore attaches from one bone to another.