c
(c) $\left| {\,\begin{array}{*{20}{c}}1&{2a}&a\\1&{3b}&b\\1&{4c}&c\end{array}\,} \right|\, = 0\,$, $[{C_2} \to {C_2} - 2{C_3}]$
==>$\left| {\,\begin{array}{*{20}{c}}1&0&a\\1&b&b\\1&{2c}&c\end{array}\,} \right| = 0$, $[{R_3} \to {R_3} - {R_2},\,{R_2} \to {R_2} - {R_1}]$
==> $\left| {\,\begin{array}{*{20}{c}}1&0&a\\0&b&{b - a}\\0&{2c - b}&{c - b}\end{array}\,} \right|\, = 0$ ; $b(c - b) - (b - a)\,(2c - b) = 0$
On simplification, $\frac{2}{b} = \frac{1}{a} + \frac{1}{c}$
$\therefore$ $ a, b, c$ are in Harmonic progression.