D for me as well.
let a =jacks rate, and b=johns rate
0.25a=2 -->a=8 -->b=12
oa is d. in this case we use the formula:
time necessary for Jack + time necessary for John = total time
let's consider the formula 1/x + 1/y = total time or xy/(x+y)=total time
in this case we already have the two times: 1/x and 1/y, which are a and b.
is that right? I encounter many problems in approaching work questions.