length of slot = 2 bits & length of frame = 0.5 ms ??
If it would be the other way instead then
let every station needs 'x' bps as bandwidth ,then in 0.5 ms with 'x' bps the station would transmit 2 bits of frame (not realistic though) which can be written as x*(0.5*10^-3)=2
it will give x= 4 kbps.
But there are 4 stations, thus link bandwidth must be at least =4*4= 16 kbps...