The correct choice is (a) 250 N

Easiest explanation: We know that in case of a Weston differential pulley block,

V.R. = 2D/D – d = 2 x 300/300 -250 = 12

Using the relation, Efficiency = M.A./V.R. x 100

or, 50 = M.A./12 x 100

M.A. = 6

Again, M.A. = W/P

6 = 1.5 x 1000/ P

P = 250 N.