The act of nullifying a contract refers to:

The act of nullifying a contract means to declare it invalid, effectively voiding any legal obligations or enforceability that the contract may initially have had. When a contract is nullified, it is treated as though it never existed, and the parties involved are released from any commitments they may have made under that agreement. This concept is important in legal contexts where parties may seek to annul contracts due to reasons such as misrepresentation, fraud, duress, or a lack of capacity to enter into a contract.

The other choices do not accurately reflect the meaning of nullification. Enforcing a contract's terms or making it legally binding refers to upholding the agreement and ensuring that it is carried out as specified, while supporting its agreement implies a positive reinforcement of the contract’s terms rather than rendering them invalid. Thus, the correct answer is centered around the fundamental legal principle of invalidating the agreement.