Explanation

(b)
$C_{n}H_{2n}O_{2}$ is general formula for open chain acid and ester.
$n = 3\ \ \ C_{3}H_{6}O_{2}$
O
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \parallel \$
Acid $\ \ \ \ CH_{3}CH_{2} - C - O - H$
O
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \parallel$
Easter $CH_{3} - C - O - CH_{3}$