Identify the code sequence: 1010 1011 1001 1000

Step 1: Check for BCD
Valid BCD codes range from 0000 to 1001 only.
Codes 1010 and 1011 are invalid in BCD.
∴ The sequence is not BCD.

Step 2: Check for Excess-3 Code
Excess-3 code = BCD + 0011

Standard Excess-3 table:

Decimal → Excess-3
5 → 1000
6 → 1001
7 → 1010
8 → 1011

Comparison with given sequence:

1010 → Excess-3 of 7
1011 → Excess-3 of 8
1001 → Excess-3 of 6
1000 → Excess-3 of 5

All given codes exactly match the standard Excess-3 representations.
∴ The sequence represents Excess-3 code.

Step 3: Why it is not Gray Code
Although only one bit changes between successive codes, Gray code does not define fixed digit representations such as 1010 for 7 or 1011 for 8.
These codes follow the standard digit mapping of Excess-3, not Gray numbering.

Conclusion:
Correct Answer → B. Excess-3