The question asks about the type of compound formed when a hydrocarbon undergoes oxidation and results in a sweet-smelling liquid. Let's analyze the possibilities based on the options provided:

1. Ketones: Ketones are generally produced by oxidizing secondary alcohols. They typically have a sharp or pungent odor rather than a sweet smell.
2. Amides: Amides usually do not produce sweet smells; they may have a slight ammoniac or fishy odor instead due to the presence of the nitrogen atom.
3. Alkenes: Alkenes are hydrocarbons with double bonds and do not generally produce a sweet smell when oxidized. They are more likely to produce aldehydes or ketones that are not characterized by a sweet aroma.
4. Esters: Esters are known for their sweet, fruity smells. They are often used in perfumes and flavorings. Esters can be formed by the oxidation of alcohols or hydrocarbons when combined with an acid component.

From the analysis, esters are the compounds most commonly associated with a sweet smell when formed. Thus, the correct answer, considering the conditions and the typical characteristics of the compounds mentioned, is:

An ester is most likely formed during the reaction.

In conclusion, based on the nature of esters and their characteristic sweet aroma, the formation of an ester is logical when a hydrocarbon undergoes oxidation in laboratory settings to produce a sweet-smelling liquid.