In foundry and ferrous metallurgy practice, select the correct statements about casting sound test, cupola usage, and hot-shortness (red-short) in iron. Choose the most comprehensive correct option.

Introduction / Context:
Quality checks and furnace selection are basic foundry topics. This question consolidates three routine facts: acoustic inspection of castings, the role of the cupola furnace, and the implications of hot-shortness for fabrication processes such as welding and forging.

Given Data / Assumptions:

• Defective castings with voids or blowholes have altered resonance.
• Cupolas are traditional shaft furnaces for cast iron.
• Hot-shortness is typically caused by sulphur, embrittling iron at red heat.

Concept / Approach:

Sound test: a homogeneous, defect-free casting rings; voids dampen vibration, producing a dull note. Cupola: widely used for continuous melting of cast iron with coke, limestone, and scrap/returns. Red-short iron: embrittled at high temperatures, cracks under hot working and has negligible welding value.

Step-by-Step Solution:

Verification / Alternative check:

Foundry manuals and QA guides describe these checks and furnace roles consistently across curricula.

Why Other Options Are Wrong:

Only (a) and (b): incomplete because it ignores the established fact about red-short iron.

Common Pitfalls:

Confusing cold-shortness (phosphorus) with hot-shortness (sulphur); assuming any dull sound arises from surface roughness rather than internal voids.

Final Answer:

All the above