The way you change the speed of a wave is to: a
Change it's medium
b
Change it's energy
c
Transfer it to a new position
d
Apply a force

transfer it to a new position

A.) Change it's medium

Explanation:

whenever a wave travels from one medium to another, then due to difference in optical density of the two mediums, the speed of wave changes.

for example :

when a wave travels from air to water, water is more optically denser hence it's speed decreases, this also leads to refraction ( bending of light )

if you already have the answer to (a), then just work out how many periods there are in 1 minute, and that's the answer to (c).

work: w = fd (given)

work done by chet   =50*1 = 50 j

work done by mika =40*2 = 80 j

work done by sara =30*1.5 = 45 j

work done by bill =60*0.5 = 30 j

students in order from the greatest amount of work done to the least is mika> chet> sara> bill

hence option (b) is correct.