Which of the following field of the TCP header tells how many bytes may be sent starting at the byte acknowledged?
Answer : Window size
The Window field is used by the receiving host to tell the sender how much data the receiver permits at the moment. This is done by sending an ACK back,
Or we can say
it indicates the size of the device's receive buffer for the particular connection. That is, window size represents how much data a device can handle from its peer at one time before it is passed to the application process.
Reference : Overview of TCP Header
It is clear that option -B