Materials and microwave component properties — match each item to its characteristic List I (Material) A. Mu-metal B. Samarium–cobalt (Sm–Co) C. Sapphire (single-crystal Al2O3) D. Ferrites List II (Property) 1. Very high magnetic permeability 2. Very high magnetic remanence (and coercivity) 3. Very high thermal conductivity 4. Very stable magnetic permeability over frequency/temperature

Introduction / Context:
In RF and microwave engineering, material choice drives performance. Magnetic components, dielectrics, and permanent magnets must be selected to meet permeability, remanence, thermal, and stability requirements. Matching common materials to their hallmark properties is foundational for filter cores, isolators, phase shifters, and precision substrates.

Given Data / Assumptions:

• Mu-metal is a soft magnetic alloy used for shielding and cores.
• Samarium–cobalt (Sm–Co) is a rare-earth permanent magnet material.
• Sapphire is single-crystal alumina used as a high-performance dielectric and heat spreader.
• Ferrites are ceramic magnetic materials used widely at RF/microwave frequencies.

Concept / Approach:
We identify the dominant, most engineer-relevant property for each material in microwave practice: very high permeability for Mu-metal; high remanence/coercivity for Sm–Co magnets; excellent thermal conductivity for sapphire; and stable permeability versus frequency/temperature for many ferrite formulations designed for microwave devices.

Step-by-Step Solution:

Verification / Alternative check:
Component catalogs: Mu-metal shielding (high μ), Sm–Co magnets (high Br, Hc), sapphire substrates (high k_th, low loss), ferrite cores/isolators (stable μ over band) confirm these pairings.

Why Other Options Are Wrong:

• Assignments swapping sapphire with permeability properties or ferrites with thermal properties contradict established material science.
• Mu-metal is not prized for thermal conductivity; Sm–Co is not used for high μ cores.

Common Pitfalls:
Confusing soft magnetic (high μ, low coercivity) with hard magnetic (high remanence). Overlooking that “alumina” ceramics exist in many forms—sapphire is single-crystal with superior thermal properties.

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