A possible ans:
Every 1 ms we hv a new frame on the line. (BW 10^6 bps and frame size 1000 b).
See it as the first bit of the frame, the first bit touches the other end at 25 ms delay.
How many frames have we pushed on the line during this 25 ms as we were pushing 1 frame each 1 ms?
25 frames on the line at a time.
To identify 25 frames, we need 5 bits.