Which of the following statements is INCORRECT about BCD adder?

A BCD adder is built by feeding two 4-bit BCD digits (plus a carry-in) into a standard 4-bit binary adder, and then applying a fixed correction rule: whenever the raw binary sum exceeds 9 (1001) or a carry is produced, 0110 (6) is added to push the result back into a valid 0-9 BCD code. This correction stage exists specifically to guarantee that the adder's output is always a legitimate BCD digit -- an output that fell outside 0-9, or was left un-re-encoded in BCD, would break the entire point of chaining BCD adders across multiple decimal digit positions in a multi-digit decimal arithmetic circuit.

Applying this to the circuit at hand: the two 4-bit digits being added contribute 8 input lines, and the carry-in from the previous decimal stage adds one more, for 9 input lines in total. The four corrected sum bits plus the carry-out to the next decimal stage give 5 output lines in total. Both of these match the standard BCD adder specification, and adding two BCD-encoded decimal digits is exactly the circuit's stated purpose. The one claim that breaks the invariant above is the assertion that the output need not be in BCD -- that directly contradicts the entire reason the correction stage exists, since the sum output is required to remain a valid BCD digit at every stage. That is the false statement the question is asking for.

• "add two decimal digits in BCD" -- matches the adder's stated design goal exactly (a true statement).

• "9 input lines" -- matches 4 + 4 + 1 carry-in (a true statement).

• "5 output lines" -- matches 4 sum bits + 1 carry-out (a true statement).

• "output need not be in BCD" -- contradicts the correction logic that forces the sum back into 0-9 BCD form whenever it overflows (the false statement being asked for).