Flip-flop state validity: Some flip-flop configurations include input combinations that result in invalid or forbidden states. Evaluate the statement.

Introduction / Context:
Not all flip-flop input combinations produce defined, stable outputs. Classic S–R latches built with NOR or NAND gates are well-known examples. Recognizing these forbidden states prevents unreliable behavior and design errors.

Given Data / Assumptions:

• NOR-based S–R: S = 1 and R = 1 is invalid.
• NAND-based S–R: S = 0 and R = 0 is invalid.
• Other types (JK, D, T) avoid many invalid states via internal gating but still have asynchronous constraints.

Concept / Approach:
An invalid state arises when both outputs are forced to the same value or when the cross-coupled feedback cannot settle deterministically. Designers prevent these combinations via input conditioning or by choosing flip-flop types that internally guard against them (e.g., JK instead of simple S–R).

Step-by-Step Solution:

Verification / Alternative check:
Standard truth tables for S–R latches explicitly mark these cases as invalid or forbidden.

Why Other Options Are Wrong:

Common Pitfalls:
Driving both S and R active; assuming outputs remain complementary under invalid inputs; not deglitching asynchronous sources that might momentarily assert both inputs.

Final Answer:
Correct