Question

What occurs in order to break the bond in a Cl2 molecule?

Answer 1

Creating bonds lowers the energy state.

Thus breaking bonds requires energy: energy is absorbed.

Energy cannot be created or destroyed.

Answer 2

The bond between the 2 Cl atoms in a Cl₂ molecule is a covalent bond. 
to break this covalent bond, energy is required. 
when new bonds form, energy is released as the bond formation makes the molecule stable. molecules with low energy levels are usually stable. 
To break the covalent bond, energy is required in other words energy is absorbed. 
therefore to break the covalent bond in Cl₂ molecule 
1)energy is absorbed