Let speed of the boat be B kmph and one way distance be "d" km.

Total distance in the journey is 2d and the respective upstream and downstream time taken are d/(B-10) kmph & d/(B+10) kmph respectively

Given average speed of the journey is 37.5kmph.

=> 2d / [d/(B-10) + d/(B+10)] = 37.5
=> 2 / [1/(B-10) + 1/(B+10)] =37.5
=> (B-10)(B+10)/B = 37.5

Now use the options and plug in values.

If you use option B and set B-10(Upstream speed)=30=>B=40 then,
at B =40 equation (1) is (40-10)(40+10)/40 =37.5