Binary Coded Decimal (BCD) — in digital electronics, BCD represents each decimal digit 0–9 independently. Each single decimal digit is encoded as which fixed-length binary pattern?

Introduction / Context:
Binary Coded Decimal (BCD) is a widely used number representation in digital systems where each individual decimal digit is encoded separately. Unlike pure binary, BCD preserves the decimal digit boundaries, which simplifies human-readable I/O in calculators, digital meters, and financial devices.

Given Data / Assumptions:

• We are encoding a single decimal digit 0–9.
• Standard BCD uses “8421” weighting per digit position.
• We are not discussing packed formats beyond the per-digit code width.

Concept / Approach:
In BCD, each decimal digit is represented by a 4-bit binary code (weights 8, 4, 2, 1). Thus 0→0000, 1→0001, …, 9→1001. Multi-digit decimal numbers are formed by concatenating these 4-bit nibbles. This differs from straight binary, where the entire number is encoded as a single binary value with place weights 2^0, 2^1, 2^2, etc. BCD keeps conversion to and from decimal trivial at the digit level and avoids rounding issues in some financial computations.

Step-by-Step Solution:

Verification / Alternative check:
Check the digit 9: 1001 is a 4-bit pattern. Digits 10–15 do not exist in decimal, so the six remaining 4-bit combinations (1010–1111) are “invalid” in basic BCD and may be unused or reserved in packed formats.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing “binary representation of the whole number” with “BCD per digit”; forgetting that some 4-bit patterns are invalid in basic BCD.

Final Answer:
4-bit binary code