The promise to refrain from doing something that a party has the legal right to do is:

a. a pledge

b. a gratuitous promise

c. forbearance

d. an illusory promise

The promise to refrain from doing something that a party has the legal right to do is c. forbearance. For example, a promise of forbearance may mean that you will renounce your right to enforce something like the payment of a debt, upholding of one's right or obligation.