Actually, the question is asking the bits needed or frames needed so as to pack the link. We don't need to think more beyond this.
Now, $1$st bit will take $25$ms to reach other host. So, to fill the link we need frames occupying the link all this while which is $25$ms. Since $1$ frame is being transmitted in $1$ms, a host need to fire at least $25$ frames at once to keep the link fully occupied. Therefore, $5$ bits needed.