Refractive index of petrofuel — what does it indicate?\nThe refractive index (n) of a petroleum fuel, defined as the ratio of light speed in air to that in the fuel, generally correlates with its ______.

Introduction / Context:
Simple optical properties such as refractive index provide rapid, non-destructive indication of hydrocarbon composition. Refineries use n and derived quantities (e.g., refractivity intercept) for quality checks.

Given Data / Assumptions:

• Comparisons among similar boiling range fuels (to avoid volatility confounding).

Concept / Approach:
Refractive index increases with increasing average molecular weight and with higher aromatic content (due to higher polarizability). Thus, n qualitatively indicates both. It is not a sulfur-specific metric. Consequently, the best answer is “Both a & b.”

Step-by-Step Solution:

Verification / Alternative check:
Empirical correlations (e.g., refractivity intercept) relate n and density to composition and average molecular weight.

Why Other Options Are Wrong:

• “Only” statements omit the dual dependence.
• “Neither” contradicts established correlations.

Common Pitfalls:
Comparing across very different boiling ranges; always compare within similar distillate cuts.

Final Answer:
Both a & b