The correct answer is option 'C' - SPDR.

SPDR stands for Serial Peripheral Data Register. It is a register that is used to store data in the master device during serial communication. Let's understand in detail why SPDR is the correct answer.

Serial Communication:
Serial communication is a method of transferring data one bit at a time, sequentially, over a single communication line. It is widely used in various devices like microcontrollers, sensors, and other peripheral devices. In serial communication, there are two devices involved - a master device and a slave device.

Master Device:
The master device is responsible for initiating and controlling the communication. It sends commands or requests to the slave device and receives data from it. The master device can be a microcontroller, a computer, or any other device capable of generating and processing serial data.

Data Transmission:
During data transmission, the master device sends data to the slave device. This data is usually stored in a register before it is transmitted. The choice of register depends on the specific microcontroller or device being used.

SPDR - Serial Peripheral Data Register:
SPDR is a register specifically designed for serial communication in microcontrollers. It is the register where the data to be transmitted is written before it is sent. The master device writes the data to be transmitted into the SPDR register, and the hardware takes care of transmitting the data bit by bit.

Advantages of using SPDR:
1. Simplifies serial communication: Using a dedicated register like SPDR for data transmission simplifies the programming and implementation of serial communication in microcontrollers.
2. Efficient data transfer: The use of SPDR ensures efficient and reliable data transfer by handling the transmission in hardware, freeing up the microcontroller's processing power for other tasks.
3. Synchronous communication: SPDR allows synchronous communication, where data is transferred in a synchronized manner with a clock signal, ensuring accurate and synchronized data transmission.

In conclusion, the data to be transmitted in the master device is written in the SPDR register. This register simplifies the process of serial communication and ensures efficient and reliable data transfer.