Definition of ROM:
ROM stands for Read-Only Memory. It is a type of memory that stores data permanently and is non-volatile, meaning the data is not lost when the power is turned off. The data stored in ROM cannot be modified or erased by normal computer operations.

Characteristics of ROM:
- ROM is non-volatile, which means it retains its data even when the power is turned off.
- The data stored in ROM is permanent and cannot be modified or erased.
- ROM is used to store firmware, boot loaders, and other software that is essential for the computer system to function.
- The data in ROM is accessed sequentially.

Types of ROM:
There are several types of ROM, including:
1. Mask ROM (MROM): This type of ROM is programmed during the manufacturing process and cannot be modified or erased.
2. Programmable ROM (PROM): This type of ROM can be programmed by the user after it is manufactured.
3. Erasable PROM (EPROM): This type of ROM can be erased and reprogrammed multiple times using ultraviolet light.
4. Electrically Erasable PROM (EEPROM): This type of ROM can be erased and reprogrammed using electrical signals.

ROM as a Combinational Circuit:
A combinational circuit is a digital circuit where the output is solely determined by the current input values. ROM can be considered as a combinational circuit because it produces an output based on the address input. When a specific address is given to the ROM, it outputs the corresponding data stored at that address. The output of a ROM depends only on the address input and not on any previous state or sequence of inputs.

ROM as a Sequential Circuit:
A sequential circuit, on the other hand, is a digital circuit where the output not only depends on the current input values but also on the previous state of the circuit. ROM does not have any internal state or memory, so it does not qualify as a sequential circuit.

Conclusion:
In conclusion, the correct answer is option 'B' - Combinational circuit. ROM is a type of memory that stores data permanently, and its output is solely determined by the current address input. It does not have any internal state or memory, which is a characteristic of sequential circuits.