Why isn't D the right answer? Because from my understanding whenever a question asks you for a horizontal asymptote, you plug in both + and - infinity.

Step-by-step explanation:
lim(x→-∞) 5x / (1 − x) = -5
lim(x→∞) 5x / (1 − x) = -5
y = 5x / (1 − x) only has a horizontal asymptote at y = -5.