The key schedule results in generating multiple keys from the one secret key. These multiple keys are used: a. in multiple sessions of communications one after the other. For example, if someone has 12 keys, they can use it for twelve video calls one after the other.
b. such that one of them is picked up at random at a time.
c. some as private keys, some as public keys.
d. for different rounds of encryption for the same plaintext to strengthen the cipher.

Option(d) is the correct answer to the given question .

Explanation:

There are various type of algorithm is used for the purpose of the key scheduling such as AES .in the AES algorithm we used same key for encryption and decryption of text .The  main objective of the AES algorithm it is used by Various round of the similar plain text encryption to reinforce the cipher text.

answer; i believe that the correct answer is (training);

click the big logo on the top left of the screen.   hope this !

-ttl