What is the percentage by mass of carbon in caproic acid?
Just over 64%.
Caproic acid is the trivial name for hexanoic acid
Molar mass is 116.16 g/mol, and that of carbon is 12.01 g/mol. The molecules contain a total of 6 carbon atoms, therefore carbon is present at (6 x 12.01)/116.16 = 64.04% by mass.