You are experimenting with a binary in /tmp/foo.d that expects its configuration file at /etc/foo.conf.
You don’t want to save it there, but use a symbolic link to /tmp/foo.d/foo.conf instead.

Which command would accomplish that?

A.
ln -s /tmp/foo.d/foo.conf /etc/foo.conf

B.
ln /tmp/foo.d/foo.conf /etc/foo.conf

C.
ln -s /etc/foo.conf /tmp/foo.d/foo.conf

D.
ln /etc/foo.conf /tmp/foo.d/foo.conf

Explanation:
To create a symbolic link (symlink) you need to specify the -s option for the command ln.
The correct syntax for ln is
ln -s <original> <symlink>