The correct answer is D. Straight chain paraffins.
Paraffins are hydrocarbons that have only single bonds between their carbon atoms. They are the simplest type of hydrocarbon and are found in petroleum. Straight chain paraffins have the highest knocking property because they have the most symmetrical structure. This means that they are more likely to form stable combustion products, which do not cause knocking.
Aromatic hydrocarbons are hydrocarbons that have a ring structure. They are more complex than paraffins and are not as likely to form stable combustion products. This means that they are more likely to cause knocking.
Olefins are hydrocarbons that have one or more double bonds between their carbon atoms. They are less complex than aromatic hydrocarbons and are not as likely to cause knocking as aromatic hydrocarbons.
Branched chain paraffins are paraffins that have one or more branches off of their main chain. They are less symmetrical than straight chain paraffins and are not as likely to form stable combustion products. This means that they are more likely to cause knocking than straight chain paraffins.