D. 4(4/9)
Feedback In this type of questions, first we need to calculate 1 hours work, then their collective work as,
A's 1 hour work is 1/8
B's 1 hour work is 1/10
(A+B)'s 1 hour work = 1/8 + 1/10 =9/40
So both will finish the work in 40/9 hours =4(4/9)