A key schedule is:

A.
A set of subkeys derived from a secret key

B.
A method of generating keys by the use of random numbers

C.
Using distributed computing resources to conduct a brute force
attack on a symmetric algorithm

D.
A list of cryptographic keys to be used at specified dates and times

Explanation:
The subkeys are typically used in iterated block ciphers. In this
type of cipher, the plaintext is broken into fixed-length blocks and
enciphered in rounds. In a round, the same transformation is
applied using one of the subkeys of the key schedule.