Answer
The buoy bobs up and down with a frequency of $0.33~Hz$
Work Step by Step
We can find the frequency at which the buoy bobs up and down:
$\lambda~f = v$
$f = \frac{v}{\lambda}$
$f = \frac{2.5~m/s}{7.5~m}$
$f = 0.33~Hz$
The buoy bobs up and down with a frequency of $0.33~Hz$
