ANS:A - Aromatics

Among the options provided, aromatics generally have the lowest cetane number. Here's why:

1. Cetane Number: The cetane number is a measure of the ignition quality of diesel fuel during compression ignition (diesel) engines. It indicates how easily the fuel ignites when injected into the combustion chamber.
2. Aromatics: Aromatic hydrocarbons, such as benzene, toluene, and xylene, typically have lower cetane numbers compared to other hydrocarbon types like paraffins (both normal and iso), naphthenes, and olefins.
3. Reason for Low Cetane Number: Aromatic hydrocarbons have a higher resistance to autoignition due to their molecular structure, which can inhibit the combustion process in diesel engines. This results in a lower cetane number, indicating a longer ignition delay and potentially less efficient combustion.
4. Other Hydrocarbons: In contrast, n-paraffins (normal paraffins), i-paraffins (iso-paraffins), naphthenes, and olefins generally have higher cetane numbers compared to aromatics.