The probability that a single bit will be in error on a typical public telephone line using 4800 bps modem is 10 to the power -3. If no error detection mechanism is used, the residual error rate for a communication line using 9-bit frames is approximately equal to

ANS:B - 0.009

Here the data rate of 4800 bps is redundant information.
Probability that a single bit is in error 10^-3 = 0.001t
Probability that a single bit is not in error = 1 - 0.001 = 0.999

In a frame of 9 bits the residual error rate value signifies the probability that at least one of the bits out of the nine is in error.

Thus, chances that all 9 bits are correct = 0.999^9 = 0.991 (approx)
Residual error rate = chances that at least one of 9 bits is incorrect = 1 - 0.991 = 0.009 (approx)