can someone explain to me how the answer is C, much thanks
The first thing to do is count the longest (parent) chain. When a carboxyl group is present, that must be counted in the parent chain. The longest parent chain (including the carboxyl group) contains 8 carbon atoms. Hence, this is octanoic acid (a number is not required to specificy the location of the carboxyl group because it must be present at the terminal carbon atom). The next thing to do is notice that we have a methyl (CH3) group coming off one of the carbons, and a propyl (CH3CH2CH3) group coming off another carbon. The important thing to do now is note what number carbon they're coming off. Counting from the carbon of the carboxyl group (i.e. Carbon one), we can see that the propyl group is coming off carbon number 5, while the methyl group is coming off of carbon 6.
So, we know three things now:
Parent chain: octanoic acid
We also have 5-propyl and 6-methyl
So, our compound:
6-methyl-5-propyl octanoic acid, C.