Input Rate =4Mbps
Output Rate Required=8 Mbps
Net effect=4Mbps drain rate of the bucket
Initial capacity =15Mb
Now to drain 15Mb at rate of 4Mb/sec,we need
4Mb drained in => 1 sec
1Mb drained in =>1/4 sec
15Mb drained in=>15/4 sec =3.75 sec
So, we can have full 8Mbps for 3.75 seconds
a is correct answer