The Term Vesting Can Best Be Described as Which of the Following

The term vesting can best be described as which of the following?

A) paying shares out to employees on a regular,usually annual basis
B) imposing equity forfeitures on cofounders over a certain period of time on a piecemeal basis should they not stay with the company
C) defining a particular time and range of duties the cofounders must contribute to the business
D) determining the investment that each cofounder must contribute to the start and growth of the business