Correct : d

If (x + y) is proportional to (x − y), then:
x + y = k(x − y)

Solving:
x + y = kx − ky
x − kx = −ky − y
x(1 − k) = −y(k + 1)

So,
x/y = (k + 1) / (k − 1)

This value depends only on the constant k, not on x or y.

Therefore, x/y is a constant.