What number must be added to in the following expression to make 1? <br> $\frac{1}{2} + \frac{1}{3} + ...... = 1$

Question

$\begin{array}{11} (a)\; \frac{1}{5} \\ (b)\; \frac{1}{6} \\ (c)\; \frac{2}{3} \\ (d)\; \frac{3}{5} \end{array}$