BCD validity check — Determine the decimal status of the single BCD nibble 1010. Is it a valid decimal digit encoding?

Introduction / Context:
Standard BCD uses a 4-bit group to encode decimal digits 0 through 9. Codes from 1010 to 1111 are not valid for a single BCD digit and are typically flagged as errors or treated as undefined in hardware decoders. This question tests recognition of invalid BCD patterns.

Given Data / Assumptions:

• Single BCD nibble presented: 1010.
• Valid BCD set is 0000–1001.
• We are not using special variants like zoned or signed BCD here.

Concept / Approach:
Because BCD mirrors decimal digits, any 4-bit value above 1001 does not represent a valid decimal digit. The pattern 1010 equals decimal 10 in pure binary, but BCD cannot encode decimal 10 as a single digit; two digits are required (1 and 0).

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

Common Pitfalls:

Final Answer: