c
(C) રેખાઓ ની કુલ સંખ્યા \( = \,\,\frac{{({n_2} - {n_1})\left[ {({n_2} - {n_1}) + 1} \right]}}{2}\,\)

તેથી \(10\,\, = \,\,\frac{{\left( {{n_2} - 1} \right)({n_2})}}{2}\,\,\,\,i.e.\,\,{n_2}^2 - {n_2} - 20\,\, = \,\,0\)

\({n_2}^2 - 5{n_2} + 4{n_2} - 20 = 0\,\,\,\)

\(\Rightarrow \,\,\,{n_2}({n_2} - 5) + 4({n_2} - 5) = 0\,\,\, \Rightarrow \,\,\,({n_2} + 4)({n_2} - 5) = 0\,\,\, \Rightarrow \,\,{n_2} = 5\)