An electron in sixth orbit of hydrogen atom deexcites to ground state in all possible ways. Number of spectral lines observed in Balmer series are

$$\begin{gathered} \mathrm{Number} \mathrm{of} \mathrm{spectral} \mathrm{lines}=\frac{\mathrm{n}\left(\mathrm{n}+1\right)}{2} \left(\mathrm{where} \mathrm{n}={\mathrm{n}}_2-{\mathrm{n}}_1\right) \\ \mathrm{Given}, \mathrm{n}=6-1=5 \\ \mathrm{Number} \mathrm{of} \mathrm{spectral} \mathrm{lines}=\frac{5\left(5+1\right)}{2} \\ =15 \\ \mathrm{Number} \mathrm{of} \mathrm{spectral} \mathrm{lines} \mathrm{in} \mathrm{Balmer} \mathrm{series}=4; \\ \mathrm{The} \mathrm{possible} \mathrm{electronic} \mathrm{transitions} \mathrm{are} 6\to 2, 5\to 2, 4\to 2, 3\to 2 \end{gathered}$$