The file /etc/ssh/ssh_host_key should be:

A.
world-readable

B.
readable to group sys

C.
readable to root only

D.
readable by all SSH users

Explanation:
From the man pages:
the host key files are normally not readable by anyone but root. The default is /etc/ssh/ssh_host_key for protocol version 1, and /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for protocol version 2.