Explanation:

In order for a computer to connect to the internet, it must have a unique IP address. The IP address serves as a unique identifier for each device connected to the internet. Without a unique IP address, it would not be possible for data to be sent and received correctly.

IP Address:
- An IP address is a numerical label assigned to each device connected to a computer network.
- It acts as an identifier for the device and allows it to communicate with other devices on the internet.
- The IP address consists of four sets of numbers separated by periods, such as 192.168.0.1.

Unique IP Address:
- Each device connected to the internet must have a unique IP address.
- This is because IP addresses are used to route data between devices.
- If multiple devices had the same IP address, data would not be able to reach the intended device.

Example:
- Imagine a scenario where two computers have the same IP address.
- When data is sent to that IP address, it would be impossible to determine which computer should receive the data.
- This would result in data loss or incorrect data delivery.

Compatibility:
- While it is not necessary for a computer to be an IBM PC to connect to the internet, it must be internet compatible.
- This means that the computer's hardware and software must support internet connectivity.
- Most modern computers, regardless of their brand or operating system, are internet compatible.

Modem Connection:
- While modems were commonly used in the past to connect computers to the internet, they are not the only means of connection.
- Nowadays, computers can connect to the internet through various methods such as Ethernet, Wi-Fi, or cellular data.
- Therefore, having a modem connection is not a requirement for internet connectivity.

Conclusion:
In conclusion, the correct answer is option 'B' - Each computer connected to the internet must have a unique IP address. This is because IP addresses are essential for routing data between devices on the internet. The other options, such as being an IBM PC or having a modem connection, are not necessary for internet connectivity.