125000 Bytes (or 125 KB) should be the right answer.
Jamming signal is like a negative acknowledgement used to inform all the nodes connected to a link that collision occurred.
Suppose a node P is transmitting frames to other node Q which is 200 km away from P.
Now suppose the frame sent by P to Q collides with another frame, at a point very near to Q after travelling 199.99 km, then to inform node P about collision, the jamming signal has to travel almost 200 km.
So in the worst case, P will get informed about collision only after two way propagation delay.
& if P receives jamming signal after it has completed its frame transmission, then will never know that which frame caused the collision & that the frame that it sent to Q has been collided & lost.
So the frame that P is going to transmit should be large enough to be transmitted until a longest two way propagation time.
That is frame transmission time must be at least as much as 2*Propagation time.
Here 2 * Propagation time = 2 * 10-3 seconds,
& Transmission rate = 500 Mega bits per second.
Let frame size be L bits.
Then (L / 500 * 106) >= 2 * 10-3
Hence L >= 106 bits or L >= 1,25,000 Bytes or L >= 125 KB.