Knock rating reference fuels for spark-ignition engines: choose the correct pair used to define the octane scale. Context: Knock intensity in SI engines is calibrated using blends of two standard reference fuels. Select the correct pair that forms the basis of the octane number scale.

Given
Octane number quantifies resistance to knock by comparing a fuel to blends of two references.

Concept/Approach
Iso-octane (2,2,4-trimethylpentane) is assigned 100 octane (high knock resistance). n-Heptane is assigned 0 octane (low knock resistance). Real fuel octane equals the % iso-octane in an iso-octane/n-heptane blend giving the same knock tendency.

Why others are wrong
Alpha-methyl naphthalene and aniline are not the modern octane references; normal hexane is not used for the octane number scale.

Final Answer
Normal heptane and iso-octane.