• Enzymology of Fatty Acid Desaturation in Mammals:
• Mammalian hepatocytes possess $\Delta^9$, $\Delta^6$, and $\Delta^5$ acyl-CoA desaturases, which can introduce double bonds between an existing double bond and the carboxyl terminal.
• Mammals completely lack $\Delta^{12}$ and $\Delta^{15}$ fatty acid desaturases:
• They cannot introduce a double bond beyond Carbon atom 9 (counted from the carboxyl terminal towards the methyl end).
• Consequently, Linoleic acid ($18:2\Delta^{9,12}$) and $\alpha$-Linolenic acid ($18:3\Delta^{9,12,15}$) cannot be synthesized de novo and are obligate dietary essential fatty acids.
• nalysis:
• (Carbon atom 9): Correct.

Correct option: (1) Carbon atom 9