If Bob wants to send a secure message to Alice using an asymmetric cryptographic algorithm, which key does he use to encrypt the message?

Alice's public key

Explanation

Based on the information provided within the question it can be said that in order to do this Bob needs to use Alice's public key. This is mainly due to the reason that Bob needs to use the key of the individual that he is writing the message to, that way only they would be able to decipher the message. Otherwise Alice would never be able to decipher the message that was meant for her.