Answer: C
In statistical TDM, the bandwidth is divided into slots each for a source if the source requires. There is no dedicated slot for each source in the bandwidth.
STDM does not reserve a time slot for each terminal, rather it assigns a slot when the terminal is requiring data to be sent or received.
Multiplexer bandwidth = 5000bits = 5 packets can be send in a time unit.
If x number of source want to transmit in a particular time unit then MBW will be divided among x sources. Therefore,
if [x <= 5] {
all x sources can successfully transmit their 1000bits because [x*1000]<=[5000]
} else {
all sources cannot transmit their complete 1000 bits and there will be backlog of x (incompletely transmitted) packets.
}
So,
6 [Backlog], 9 [Backlog], 3 [-], 7 [Backlog], 2 [-], 2 [-], 2 [-], 3 [-], 4 [-], 6 [Backlog], 1 [-], 10 [Backlog], 7 [Backlog], 5 [-], 8 [Backlog], 3 [-], 6 [Backlog], 2 [-], 9 [Backlog], 5 [-]
Average number of backlogged packets per time unit during the given period is = [6+9+7+6+10+7+8+6+9]/20 = 3.4