Which logic element uses a control relay?

• A. And gate
• B. Or gate
• C. Memory element
• D. Not gate

Answer: (C)

The concept here is the difference between a memory (sequential) element and purely combinational logic. A memory element stores information and can hold its output even after the input changes, which is achieved by latching with a relay. In relay logic, a control relay is used to set or reset that stored state, effectively “remembering” that condition until something else changes it. That ability to hold a state is what makes a memory element dependent on a control relay.

The other options perform immediate logic based on current inputs only; they don’t store or recall a previous state, so they don’t rely on a control relay to maintain output. Therefore, the memory element is the part that uses a control relay to remember and maintain its state.