Explanation

(c)
Let $S_{n} = \frac{1}{2.5} + \frac{1}{5.8} + \frac{1}{8.11} + ... + \frac{1}{(3n - 1)(3n + 2)}$
$= \frac{1}{3}\left\lbrack \frac{1}{2} - \frac{1}{5} + \frac{1}{5} - \frac{1}{8} + .\ldots.. + \frac{1}{3n - 1} - \frac{1}{3n + 2} \right\rbrack$
$= \frac{1}{3}\left\lbrack \frac{1}{2} - \frac{1}{3n + 2} \right\rbrack = \frac{n}{6n + 4}$