What is a purpose of an SSH host key?

 An SSH host key is a cryptographic key used for authenticating computers in the SSH protocol. Host keys are key pairs, typically using the RSA, DSA, or ECDSA algorithms. Public host keys are stored on and/or distributed to SSH clients, and private keys are stored on SSH servers. Each host (i.e., computer) should have a unique host key. Host keys are used for authentication towards the connecting client, analogous to user SSH keys. Host keys are generated using asymmetric encryption algorithms like RSA, DSA, or ECDSA algorithms12. When a client connects to the host, the host sends its public host key to the client, and the client verifies that the host key matches the one stored in its known hosts file. If the host key is unknown or has changed, the client will display a warning and prompt the user to accept or reject the host key. This is to prevent man-in-the-middle attacks, where an attacker intercepts the connection and pretends to be the legitimate host. The other options are either incorrect or irrelevant to the purpose of an SSH host key. References:

What is an SSH Host Key & How are They Configured?, What are SSH Host Keys? section

SSH Host Key Management Demystified, What are SSH host keys? section

What is SSH host key - omnisecu.com, first paragraph