The correct answer is option 'C' - cetane and n-heptane.

Explanation:
Cetane rating is a measure of the ignition quality of diesel fuel. It indicates how easily the fuel ignites and how smoothly it burns in a diesel engine. The cetane rating of a fuel is determined by comparing its ignition characteristics to those of reference fuels.

The two reference fuels used for cetane rating are cetane and n-heptane. Here's why:

1. Cetane:
- Cetane is a pure hydrocarbon compound with a chemical formula of C16H34.
- It has excellent ignition characteristics, meaning it ignites easily and burns smoothly.
- Cetane is assigned a cetane rating of 100, which is the highest possible rating.
- Fuels with a higher cetane rating have better ignition quality and combustion efficiency.

2. n-Heptane:
- n-Heptane is a pure hydrocarbon compound with a chemical formula of C7H16.
- It has poor ignition characteristics, meaning it ignites with difficulty and burns less smoothly.
- n-Heptane is assigned a cetane rating of 0, which is the lowest possible rating.
- Fuels with a lower cetane rating have poorer ignition quality and combustion efficiency.

By comparing the ignition characteristics of a test fuel to those of cetane and n-heptane, its cetane rating can be determined. The test fuel is compared to cetane and n-heptane in a standardized engine test called the Cetane Engine Test.

In this test, the test fuel is injected into a combustion chamber and ignited. The ignition delay, which is the time between fuel injection and the start of combustion, is measured. The ignition delay of the test fuel is then compared to the ignition delays of cetane and n-heptane under the same conditions.

If the test fuel has an ignition delay similar to cetane, it will have a high cetane rating. If the test fuel has an ignition delay similar to n-heptane, it will have a low cetane rating.

So, the two reference fuels used for cetane rating are cetane and n-heptane, as they represent the extremes of ignition quality for diesel fuel.