If we consider 100 units of work as total work. (To avoid fractions :3)
Then in 1 hour one machine does (100/4) = 25 units of work, while another one does (100/2)=50 units of work.
They together do 75 units of work in 1 hour ,so 100 units of work is done in (100/75) hours or 4/3 hours
So, (C)4/3 is the answer