for this question you need to use the equation Y = C + I
its pretty straight forward from there...
Y = C + 40
Y = (100 + 0.8Y) + 40
then its just maths...
Y = 140 + 0.8Y
Y - 0.8Y = 140
Y(1 - 0.8) = 140
Y(0.2) = 140
:. Y = 140/0.2
= 700
Therefore, the answer is D, 700