In programmable controllers (PLC context; original text used “PC”), what are the two fundamental types of relays used for logic and memory behavior? Select the correct pair.

Introduction / Context:
In programmable controller (PLC) terminology, “relays” commonly refer to internal output coils or memory bits that mimic the behavior of hardware relays for logic control. Two core behaviors are especially important: whether the coil state is preserved through power cycles (retentive) or cleared when power or certain conditions are removed (non-retentive). Understanding this distinction helps designers choose safe defaults and predictable restart behavior.

Given Data / Assumptions:

• The original prompt says “PC,” but by standard industrial usage and the options provided, this is clearly about PLCs.
• We compare internal coil behaviors, not only physical output modules.
• We want the canonical two types used when discussing memory retention semantics.

Concept / Approach:
Non-retentive coils lose their energized state on power loss, CPU stop, or program download. They are ideal where a “safe off” condition is required on restart. Retentive coils preserve their last state across power interruptions or mode changes (subject to PLC settings), which is helpful for latching modes, counts, or states that must not be forgotten. Good design documents explicitly state which tags are retentive to avoid unexpected behavior after a reboot.

Step-by-Step Solution:
Identify common PLC coil types by behavior: retentive vs non-retentive.Exclude role-based labels that do not define retention (e.g., “output relay,” “control relay”).Select the pair that distinguishes memory persistence: retentive and non-retentive.

Verification / Alternative check:
Vendor manuals (for example, Allen-Bradley, Siemens, Omron) consistently document retentive vs non-retentive memory bits/coil instructions, confirming the two-type framing used by practitioners.

Why Other Options Are Wrong:
“Output relay” and “control relay” describe function or wiring context, not retention behavior. “All of the above” is incorrect because only one pair captures the core behavioral dichotomy.

Common Pitfalls:
Assuming all coils are retentive by default; in many PLCs, retentiveness must be configured. Also, confusing physical relay modules with internal “software relays.”

Final Answer:
A retentive relay and a non-retentive relay.