Introduction:
In computer systems, data is stored in various storage devices such as RAM, hard drives, and solid-state drives. When the CPU needs to access this data, it can either read or write it. Reading refers to retrieving data from a storage device, while writing involves storing new or modified data back to the storage device.

Explanation:
The given question asks about a read bit, which means accessing data from a storage device. The question suggests that a read bit can be read by the CPU and written by the peripheral. Let's analyze each option to understand the possibilities.

a) Read bit can be read and written by CPU:
This option suggests that the CPU can both read and write the read bit. However, this is not accurate because the read bit is typically read by the CPU but not written by it.

b) Read bit can be read and written by peripheral:
This option implies that the peripheral device can both read and write the read bit. While peripherals can write data to storage devices, they do not typically read data directly from a read bit.

c) Read bit can be read by peripheral and written by CPU:
This option suggests that the peripheral device can read the read bit, and the CPU can write it. This is not entirely accurate because the read bit is usually read by the CPU and not by the peripheral device.

d) Read bit can be read by CPU and written by the peripheral:
This option accurately describes the functionality of a read bit. The CPU is responsible for reading data from storage devices, including the read bit. However, when it comes to writing data, the CPU can delegate this task to a peripheral device, which can write the read bit or any other data back to the storage device.

Conclusion:
Among the given options, option 'D' correctly states that a read bit can be read by the CPU and written by the peripheral. The CPU is primarily responsible for reading data, while peripherals can assist in writing data back to the storage device.