Among hydrocarbons with the same carbon number, aromatics characteristically show the highest value of which property relevant to gasoline performance and knock resistance?

Introduction:
Fuel quality metrics depend strongly on molecular structure. In the gasoline range, knocking resistance is captured by octane number. Comparing hydrocarbon families at constant carbon number highlights how structure affects ignition behavior in spark-ignition engines.

Given Data / Assumptions:

• Carbon number is held constant to isolate structural effects.
• Families considered: paraffins (normal/branched), naphthenes (cycloalkanes), and aromatics.
• We focus on gasoline-relevant anti-knock property.

Concept / Approach:
Aromatics exhibit high octane because ring conjugation and stability hinder low-temperature chain-branching reactions that cause knock. Straight-chain paraffins are most knock-prone (low octane), branched paraffins are better, naphthenes are intermediate, and aromatics are typically highest at the same carbon number. Note that high octane does not imply good diesel performance; aromatics generally have low cetane.

Step-by-Step Solution:
1) Identify the property tied to SI knock resistance: octane number.2) Rank families at equal carbon number: aromatics > isoparaffins > naphthenes > normal paraffins.3) Select “octane number” as the property in which aromatics are highest.

Verification / Alternative check:
Gasoline blending practice leverages reformate (aromatic-rich) for octane, whereas straight-run naphtha (paraffinic) needs upgrading via reforming/isomerisation to raise octane.

Why Other Options Are Wrong:
Smoke point is relevant to kerosene/jet and decreases with aromatics.Cetane number applies to diesel and is lowered by aromatics.Viscosity/freezing point are not the defining gasoline knock metrics.

Common Pitfalls:
Transferring diesel metrics (cetane) to gasoline contexts; the fuels operate on different combustion principles (CI vs SI).

Final Answer:
Octane number