The ester derivatives of the industrial chemical 'acid of butter' are widely used as food and perfume additives. The 'acid of butter' is also formed in the body when 1-butanol is completely metabolized. Facilitated by an enzyme, 1-butanol is converted to butanal, which is further converted to acid. If the 'acid of butter' is simply the oxidized form of 1-butanol, what is the structure of the acid?

• A.
• B.
• C.
• D.

Answer: (D)