c.
The PERIOD of oscillations is 4 seconds. Check out the units! Since we have units of seconds they may as well be related to period, not frequency!
We than use:
f=1/P
or
f=1/4sec
which is
f=0.25 1/sec
or 0.25 Hz (same thing!)
Note: This is one out of two questions (The other being 5) in which some math was required.