(c)
$\tan^{- 1}{\left( \frac{c_{1} - y}{c_{1}y + x} \right) + \tan^{- 1}{\left( \frac{c_{2} - c_{1}}{1 + c_{2}c_{1}} \right) + \tan^{- 1}{\left( \frac{c_{3} - c_{2}}{1 + c_{3}c_{2}} \right) + ... + \tan^{- 1}\frac{1}{c_{n}}}}}$
$= \tan^{- 1}{\left( \frac{\frac{x}{y} - \frac{1}{c_{1}}}{1 + \frac{x}{y} \bullet \frac{1}{c_{1}}} \right) + \tan^{- 1}{\left( \frac{\frac{1}{c_{1}} - \frac{1}{c_{2}}}{1 + \frac{1}{c_{1}c_{2}}} \right) + \tan^{- 1}{\left( \frac{\frac{1}{c_{2}} - \frac{1}{c_{3}}}{1 + \frac{1}{c_{2}c_{3}}} \right) + ... + \tan^{- 1}\frac{1}{c_{n}}}}}$
$= \tan^{- 1}{\ \frac{x}{y} - \tan^{- 1}{\ \frac{1}{c_{1}} + \tan^{- 1}{\ \frac{1}{c_{1}} - \tan^{- 1}{\ \frac{1}{c_{2}} + \tan^{- 1}{\ \frac{1}{c\_ 2}}}}}}$
$- \tan^{- 1}\frac{1}{c_{3}} + ... + \tan^{- 1}{\frac{1}{c_{n - 1}} - \tan^{- 1}{\frac{1}{c_{n}} + \tan^{- 1}\frac{1}{c_{n}}}}$
$= \tan^{- 1}\left( \frac{x}{y} \right)$