When using SysV init system, the file /etc/rc3.d/S55sshd is a symbolic link. Type the full path and file name to which this is linked.

Answer: /etc/init.d/sshd

Explanation:
See http://www.debian.org/doc/debian-policy/ch-opersys.html
The /etc/init.d directory contains the scripts executed by init at boot time and when the init state (or “runlevel”) is changed (see init(8)). […] These scripts are referenced by symbolic links in the /etc/rcn.d directories. When changing runlevels, init looks in the directory /etc/rcn.d for the scripts it should execute, where n is the runlevel that is being changed to, or S for the boot-up scripts.