Hydrofining (a hydroprocessing step using hydrogen and a catalyst) is widely regarded as one of the most recent and highly effective refinery methods—identify the principal purpose it serves among common options such as sulphur removal, smoke point improvement, breathing loss reduction, or viscosity-index improvement.

Introduction / Context:
Hydrofining is a refinery hydrotreating operation in which oil is contacted with hydrogen over a catalyst to upgrade quality. The core aim is to remove reactive hetero-atoms and stabilize the product. This question asks you to pinpoint the single most important objective among several plausible refinery improvements.

Given Data / Assumptions:

• Process: hydrofining (hydrotreating under H2 with catalyst)
• Candidate effects: sulphur removal, smoke point improvement, breathing loss reduction, viscosity-index (VI) improvement
• Feed types: naphtha, kerosene, diesel, VGO, etc., as typically hydrotreated

Concept / Approach:
Hydrofining promotes hydrogenolysis and hydrogenation reactions. Hetero-atoms (S, N, O) are converted to H2S, NH3, and H2O and removed. Olefins and certain aromatics are hydrogenated, improving stability and sometimes smoke point. However, among the listed outcomes, deep sulphur removal (hydrodesulphurization, HDS) is the defining purpose that enables fuels to meet sulphur specifications and emissions norms.

Step-by-Step Solution:

Verification / Alternative check:
Fuel regulations (e.g., low-sulphur diesel/gasoline) are met by hydrotreaters designed primarily for sulphur removal; other properties improve secondarily.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing secondary property improvements with the primary compliance-driven purpose (sulphur removal).

Final Answer:
Removal of sulphur