Answer
B should disassemble $\mathrm{M}(5)$ and check that it was correctly transmitted.
If so, B should send an $\mathrm{ACK}(5)$ message to $\mathrm{A} ;$ if not, $\mathrm{B}$ should discard
the faulty message and wait for $\mathrm{A}$ to resend it. Although this message
has been received out of sequence (presumably, because $\mathrm{M}(4)$ was lost)
no special action is required at this time; $\mathrm{B}$ did not send a $\mathrm{ACK}(4)$ so $\mathrm{A}$
should in time resend $\mathrm{M}(4) .$
Work Step by Step
B should disassemble $\mathrm{M}(5)$ and check that it was correctly transmitted.
If so, B should send an $\mathrm{ACK}(5)$ message to $\mathrm{A} ;$ if not, $\mathrm{B}$ should discard
the faulty message and wait for $\mathrm{A}$ to resend it. Although this message
has been received out of sequence (presumably, because $\mathrm{M}(4)$ was lost)
no special action is required at this time; $\mathrm{B}$ did not send a $\mathrm{ACK}(4)$ so $\mathrm{A}$
should in time resend $\mathrm{M}(4) .$
