Match radio/ionospheric terms to their definitions: (A) Penetration frequency, (B) Optimum working frequency (OWF), (C) Gyro-frequency, (D) Maximum usable frequency (MUF).

Introduction / Context:
This question checks foundational knowledge in ionospheric radio propagation. You must distinguish among penetration frequency, optimum working frequency (OWF), gyro-frequency, and maximum usable frequency (MUF)—terms that determine whether, and how well, radio waves traverse or are refracted by the ionosphere.

Given Data / Assumptions:

• Penetration frequency: associated with the critical frequency concept.
• OWF: frequency that gives the most reliable ionospheric link.
• Gyro-frequency: electron gyration in a magnetic field.
• MUF: the highest frequency that returns to Earth for a given path and conditions.

Concept / Approach:
Map each term to classic textbook definitions used in shortwave/skywave communication: MUF sets the upper bound for refraction; OWF is a practical sweet spot below MUF; gyro-frequency depends on the geomagnetic field; penetration frequency is tied to the critical frequency beyond which a wave penetrates instead of reflecting in vertical incidence.

Step-by-Step Solution:

Verification / Alternative check:
Operational practice: HF operators choose OWF ≈ 0.85 * MUF to balance reliability and fading; gyro-frequency is orders of magnitude below HF in the ionosphere; critical/penetration frequency determines whether vertical rays pass through.

Why Other Options Are Wrong:

• A-4, B-1, C-3, D-2: Confuses path limits and magnetic phenomena.
• A-1, B-3, C-2, D-4: Swaps MUF and OWF; misstates gyro/critical relations.

Common Pitfalls:

• Using MUF directly as the operating frequency; links are more stable a bit below MUF.
• Confusing critical (vertical) with oblique incidence behavior.

Final Answer:
A-2, B-4, C-1, D-3