Concept:The octane rating of petrol is measured by comparing its knocking tendency with that of a standard reference fuel called iso-octane.Explanation:Iso-octane is the branched-chain alkane 2,2,4-trimethylpentane.It is assigned an octane rating of 100 and is used as the upper standard for petrol quality.A fuel with a higher octane rating burns more smoothly and is less prone to engine knocking.Therefore, the compound that determines the octane rating of petrol is iso-octane, which is 2,2,4-trimethylpentane.Answer:D. 2,2,4-trimethylpentane.