Identify the highest-melting oxide refractory among the options (noting that such materials are selected for ultra-high temperature linings).

Introduction / Context:
Oxide refractories differ widely in melting points. For extremely high-temperature applications, the highest-melting oxides are especially relevant. This question checks recognition of the relative ranking among common oxide refractories.

Given Data / Assumptions:

• Typical melting points: alumina ~2050 °C, magnesia ~2800+ °C, zirconia ~2700+ °C, thoria ~3300 °C (very high).
• Variations exist by purity and measurement method but relative order is consistent.

Concept / Approach:
Among the listed oxides, thoria (ThO2) has one of the highest melting points and is known for exceptional refractoriness. Although use is restricted for practical and regulatory reasons, in pure thermodynamic terms it outranks the others for melting temperature.

Step-by-Step Solution:
Compare the typical melting points of each candidate.Identify ThO2 as the highest among the options.Select “Thoria (ThO2).”

Verification / Alternative check:
Reference data confirm the extreme refractoriness of thoria compared with alumina, magnesia, and zirconia.

Why Other Options Are Wrong:
Al2O3/ZrO2/MgO: high but not as high as ThO2.

Common Pitfalls:
Memorizing single-point values; focus on the relative ranking for exam purposes.

Final Answer:
Thoria (ThO2)