Define the aniline point and interpret what it indicates for diesel and lubricating oils with respect to aromatic content and solvency behavior.

Introduction:
The aniline point is a classical petroleum test—temperature at which equal volumes of aniline and the oil are completely miscible. It is widely used for middle distillates and lubricant base stocks to infer solvency and aromaticity trends.

Given Data / Assumptions:

• Oil types: diesel and lube base oils.
• Aniline point correlates inversely with aromatic content (lower aniline point → higher aromatics).

Concept / Approach:
Because aniline is an aromatic amine, aromatics in oil dissolve it more readily at lower temperatures. Thus, lower aniline point implies higher aromaticity/solvency; higher aniline point implies more paraffinic character.

Step-by-Step Solution:

Verification / Alternative check:
Refining property tables relate cetane/sooting/solvency tendencies with aniline point for distillate fuels and lube stocks.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing aniline point with octane/cetane directly; it is an indirect compositional/solvency indicator, not a performance index by itself.

Final Answer:
Both (a) and (b)