For collision to be detected, the frame size should be such that the transmission time of the frame should be greater than twice the propagation delay (So, before the frame is completely sent, any possible collision will be discovered).
So, $\dfrac{1250\times 8}{10^8} \geq\dfrac{2\times 1}{ x}$
$\implies x = 2 \times 10^4 = 20000$
Correct Answer: $D$