Which statement best describes the operation of a transmission gate–based level sensitive D latch in digital circuit design?

Introduction / Context:
D latches are basic storage elements used in digital circuits to hold a single bit of data. A transmission gate based D latch implements this function using CMOS transmission gates as controlled bidirectional switches. Understanding how a level sensitive D latch works is important for learning how flip flops and registers are built in hardware.

Given Data / Assumptions:

• The device is a D latch, not a full edge triggered flip flop.
• The latch is implemented using transmission gates controlled by an enable or clock signal.
• We assume the latch is level sensitive, not edge triggered.

Concept / Approach:
A level sensitive D latch passes input D to output Q whenever the enable signal is at the active level, often logic high. During this transparent phase, changes at D appear at Q. When the enable signal goes to the inactive level, the latch stops passing the input and holds or latches the last value that was present at the moment the gate turned off. Transmission gates act as electronic switches controlled by complementary signals, allowing or blocking data flow while maintaining low resistance and good voltage levels. The description that matches this behavior is the correct one.

Step-by-Step Solution:
Step 1: Recall that transmission gate latches are level sensitive. When the enable is high, the latch is transparent.Step 2: When the enable goes low, the path from D to Q is turned off, and internal feedback maintains the stored value.Step 3: Note that this is different from an edge triggered flip flop, which samples only at clock edges.Step 4: Match this operation to the option that explicitly mentions transparent behavior when enable is active and latching when enable becomes inactive.

Verification / Alternative check:
In circuit diagrams, a transmission gate based D latch often shows a pair of CMOS transmission gates driven by complementary enable and inverted enable signals. Simulation or timing diagrams confirm that while the enable is high, output Q tracks input D with some delay, and when the enable goes low, Q holds its last value. There is no edge triggered behavior because the latch responds to the level of the control signal, not just a transition.

Why Other Options Are Wrong:
Option B: Describes an edge triggered flip flop, not a level sensitive latch, and therefore does not match the specified circuit.Option C: Suggests analog storage only, but digital latches are designed to hold distinct logic 0 and logic 1 values even though the underlying physics is analog.Option D: Claims the latch works without any enable or clock, which contradicts the definition of a controlled storage element.

Common Pitfalls:
A frequent confusion is between latches and flip flops. Latches are level sensitive, while flip flops are edge triggered. Another pitfall is ignoring the role of feedback in holding the stored value when the transmission gate is off. Understanding these differences helps when analyzing timing issues such as transparency windows and potential race conditions in sequential logic.

Final Answer:
The correct answer is It transparently passes the D input to Q while the enable signal is high and latches the last value when the enable goes low, using CMOS transmission gates as bidirectional switches.