In IS-IS (Intermediate System to Intermediate System) routing, each router is identified by a unique ISO (International Organization for Standardization) address, also known as a Network Entity Title (NET). The NET consists of three parts:
1. **Area Identifier**: Indicates the area to which the router belongs.
2. **System Identifier**: Uniquely identifies the router within the area.
3. **NSAP Selector (NSEL)**: Typically set to 00 for a router, indicating the Network Service Access Point.
The format of the ISO address is `49.XXXX.YYYY.YYYY.ZZZZ.ZZZZ.00`, where:
- `49` is the AFI (Authority and Format Identifier) indicating a private address.
- `XXXX` is the Area Identifier.
- `YYYY.YYYY.YYYY` is the System Identifier.
- `ZZZZ.ZZZZ` is the NSAP Selector.
Given the address `49.0001.00a0.c96b.c490.00`:
- **Area Identifier**: `49.0001`
- **System Identifier**: `00a0.c96b.c490`
- **NSAP Selector**: `00`
**Explanation**:
- **A. 00a0.c96b.c490 is the system identifier**:
- Correct. The System Identifier in an ISO address is a 48-bit (6-byte) field used to uniquely identify the router. In this address, `00a0.c96b.c490` is the correct 6-byte System Identifier.
- **B. 0001.00a0.c96b.c490 is the system identifier**:
- Incorrect. This includes the Area Identifier as part of the System Identifier, which is not correct.
- **C. c96b.c490 is the system identifier**:
- Incorrect. This is only part of the System Identifier. The full System Identifier must be 6 bytes long.
- **D. c490 is the system identifier**:
- Incorrect. This is an incomplete and incorrect part of the System Identifier.
**Conclusion**:
The correct part of the address that represents the System Identifier is:
**A. 00a0.c96b.c490 is the system identifier.**
**Reference**:
- Juniper Networks Documentation on IS-IS: [IS-IS Configuration](https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/isis-configuring.html)
- ISO/IEC 10589, the IS-IS routing protocol standard.