At the same temperature, compare the viscosities of common refinery products and pick the one with the highest viscosity among naphtha, fuel oil, light diesel oil (LDO), and petrol (gasoline).

Introduction / Context:
Viscosity is a fundamental property controlling pumping, atomization, and combustion behavior of fuels. Lighter fractions such as petrol (gasoline) and naphtha have very low viscosity, whereas heavier products such as diesel and residual fuel oils exhibit significantly higher viscosities and often require heating for storage and handling. The question asks you to identify, at the same temperature, which listed product has the greatest viscosity.

Given Data / Assumptions:

• Comparison is made at one common temperature.
• Products span a wide boiling range: petrol/naphtha (light), LDO (middle), fuel oil (heavy).
• No pour-point depressants or special additives assumed.

Concept / Approach:
Viscosity generally increases with average molecular weight and decreases with temperature. Residual and heavy fuel oils contain large, complex molecules (asphaltenes/resins) and thus show high viscosities compared to middle distillates (diesel/LDO) and light ends (naphtha/petrol). Consequently, heavy fuel oils often need preheating before pumping or atomization in burners, a requirement not shared by lighter fractions.

Step-by-Step Solution:

Verification / Alternative check:
Viscosity-grade specifications for fuel oils (e.g., ISO 8217 grades) are far above those of LDO or gasoline; operating manuals specify heater set points for heavy fuel lines.

Why Other Options Are Wrong:

Common Pitfalls:
Comparing values at different temperatures; keep the comparison at a fixed temperature to avoid confusion.

Final Answer:
Fuel oil