c
(c) As the system of equations has a non-trivial solution
$ \Rightarrow $ $\left| {\,\begin{array}{*{20}{c}}a&1&1\\1&b&1\\1&1&c\end{array}\,} \right|\, = \,0$
$ \Rightarrow $ $\left| {\,\begin{array}{*{20}{c}}a&1&1\\{1 - a}&{b - 1}&0\\{1 - a}&0&{c - 1}\end{array}\,} \right|\, = 0,$
by $\begin{array}{l}{R_2} \to {R_2} - {R_1}\\{R_3} \to {R_3} - {R_1}\end{array}$
$ \Rightarrow $ $a\,(b - 1)\,(c - 1) - 1\,.\,(1 - a)\,(c - 1)$$ - 1\,.\,(1 - a)\,(b - 1) = 0$
$ \Rightarrow $ $\frac{a}{{1 - a}} + \frac{1}{{1 - b}} + \frac{1}{{1 - c}} = 0$
$ \Rightarrow $ $\frac{1}{{1 - a}} - 1 + \frac{1}{{1 - b}} + \frac{1}{{1 - c}} = 0$
$ \Rightarrow $ $\frac{1}{{1 - a}} + \frac{1}{{1 - b}} + \frac{1}{{1 - c}} = 1$.