Answer:
contract
Step-by-step explanation:
A contract is a form of enforceable agreement between two parties that are legally binding. Both parties in the contract have to do the responsibilities/promises written there and any breach for the agreement can result as enforcement from the court. While a verbal contract is possible in some law, it's really hard to be proven so it's better to have a written contract instead.
Not everything can be put into a contract. If the contract against public policy, the contract will be illegal and an agreement is written there will be invalid.