Which of the following will be legally binding despite lack of consideration?

a. An employer’s promise to make a cash payment to a deceased employee’s family in recognition of the employee’s many years of service.

b. A promise to donate money to a charity on which the charity relied in incurring large expenditures.

c. A modification of a signed contract to purchase a parcel of land.

d. A merchant’s oral promise to keep an offer open for sixty days.