The correct answer is option 'A' - RTS.

Explanation:
Signal control is an important aspect in data communication, as it helps in managing the flow of data between devices. In serial communication, there are several control signals that are used to control the flow of data. Among these signals, RTS (Request to Send) is responsible for controlling the flow of data.

1. RTS (Request to Send):
- RTS is a control signal that is used by the sending device to inform the receiving device that it is ready to send data.
- When the sending device wants to transmit data, it sets the RTS line to high, indicating that it is ready to send data.
- The receiving device, upon receiving the RTS signal, acknowledges it by sending a CTS (Clear to Send) signal back to the sender.
- The sender can then start transmitting the data.
- RTS is typically used in hardware handshaking protocols like RTS/CTS (Request to Send/Clear to Send), which ensure that both the sender and receiver are ready to exchange data.

2. DTR (Data Terminal Ready):
- DTR is another control signal used in serial communication.
- It is used to indicate that the device is ready to communicate and is typically set to high when the device is powered on.
- Unlike RTS, DTR is not directly involved in controlling the flow of data.
- It is mainly used for device initialization and configuration purposes.

Conclusion:
In summary, the RTS (Request to Send) signal is responsible for controlling the flow of data in serial communication. It is used by the sending device to indicate that it is ready to transmit data. On the other hand, DTR (Data Terminal Ready) is not directly involved in controlling the flow of data but is used for device initialization. Hence, the correct answer is option 'A' - RTS.