Detecting invalid BCD digits Which 4-bit pattern below is not a valid Binary-Coded Decimal (BCD) code for a single decimal digit?

Introduction / Context:BCD encodes each decimal digit (0–9) as a 4-bit binary value. Recognizing valid versus invalid 4-bit patterns is essential for input validation, display drivers, and error checking in digital systems that use BCD instead of pure binary.

Given Data / Assumptions:

• Valid BCD digit range: 0000 through 1001 (0 to 9).
• Invalid patterns are any 4-bit values from 1010 to 1111.
• We evaluate each option against these ranges.

Concept / Approach:Check whether the given 4-bit code is less than or equal to 1001. If it exceeds 1001, it cannot represent a decimal digit in BCD and is therefore invalid.

Step-by-Step Solution:

Verification / Alternative check:List all invalid BCD codes: 1010, 1011, 1100, 1101, 1110, 1111. 1101 is in this set.

Why Other Options Are Wrong:

• 0011, 0101, 1001: all lie within 0000–1001 and therefore are valid BCD digits.

Common Pitfalls:Confusing BCD with hexadecimal, where 1010–1111 would be valid (A–F). In BCD, those patterns are illegal for single digits.

Final Answer:1101