Answer:
(c) 10m/s
Step-by-step explanation:
to find the speed of the waves you can use the following formula:

λ: wavelength of the wave
T: period
the wavelength is the distance between crests = 48m
the period is the time of a complete oscillation of the wave. In this case you have that the float takes 2.4 s to go from its highest to the lowest point. The period will be twice that time:
T = 2(2.4s)=4.8s
by replacing you obtain:

the answer is (c) 10m/s