The TXD (Transmitted Data Output) pin is a crucial component in serial communication. It carries a serial stream of transmitted data bits from a device to another device. In addition to the data bits, the TXD pin also carries other important bits that are necessary for successful communication. These additional bits include the start bit, stop bit, and parity bit.

Start Bit:
- The start bit is the first bit of the data frame transmitted over the TXD pin. It is always a logic low (0) level and serves to indicate the beginning of a new data frame.
- The start bit prepares the receiving device to synchronize with the transmitted data and prepares it to receive the actual data bits.

Stop Bit:
- The stop bit is the last bit of the data frame transmitted over the TXD pin. It is always a logic high (1) level and serves to indicate the end of a data frame.
- The stop bit provides a small amount of time for the receiving device to prepare for the reception of the next data frame.

Parity Bit:
- The parity bit is an optional bit that can be added to the data frame for error detection. It is used to check the integrity of the transmitted data.
- There are different types of parity checks, such as even parity and odd parity. The parity bit is set to a value that ensures the total number of 1s in the data frame (including the parity bit) is either even or odd, depending on the type of parity used.
- The receiving device checks the received data frame and compares it to the parity bit. If they don't match, an error in transmission is detected.

All of the Mentioned:
- The correct answer is option 'D', which states that the TXD pin carries all of the mentioned bits (start bit, stop bit, and parity bit) along with the data bits.
- This means that when data is transmitted over the TXD pin, it includes the start bit, stop bit, and parity bit in addition to the actual data bits.
- These bits are essential for proper synchronization, framing, and error detection in serial communication.

In conclusion, the TXD pin carries the complete data frame, including the start bit, stop bit, parity bit, and data bits. This ensures proper communication and enables error detection in serial communication systems.