If Bob receives an encrypted reply message from Alice, whose private key is used to decrypt the received message?

Bob's private key will be used to decrypt the encrypted reply sent from Alice because Bob had sent the encrypted message with his private key to Alice.

Explanation:

Encryption is a kind of encoding process of any data that is the only machine understood not human understood and it is used by the sender's end.

Decryption a kind of decoding process of any data which is also machine understood and used by the receiver's end.

Encryption and decryption are used for any sensitive data which will be encoded and decoded using a private key.