Structure of Propanone is:

The prefix "one" is utilized here. Thus, it implies the existence of the functional group known as ketone.
The name of the compound's parent alkyl chain is now used in the IUPAC nomenclature, followed by the required prefixes and suffixes. There are no substituted groups on the parent alkyl chain because there is no prefix in this case. The term "prop" denotes the presence of the parent alkyl chain, which has three carbon atoms in total and is arranged in a straight line.
A molecule must have two alkyl groups connected to the carbonyl carbon in order to be classified as a ketone. Propanone's structure can only be set up by bonding two methyl groups together with a carbonyl group.

Propanone contains three carbons atoms (prop). The functional group present is a ketone. 

Hence, option A is correct.