Final answer:
It is true that an FTP client can respond with a smaller window size in the TCP header to manage the flow of incoming data from the FTP server, often due to buffer or processing constraints.
Step-by-step explanation:
The FTP client can indeed respond to the FTP server with a smaller window size in the TCP header. This statement is true. In TCP, the window size is used for flow control, and it specifies the amount of data that the sender is allowed to transmit without receiving an acknowledgment from the receiver. This mechanism allows for efficient and reliable data transfer over the network. When the FTP client acknowledges the receipt of data packets from the FTP server, it may notify the server of a smaller window size if it needs to reduce the flow of incoming data, for instance, due to limited buffer space or other processing constraints.