c
(c) Let first term =$ A $ and common difference =$ D$
$\therefore a = A + (p - 1)D$, $b = A + (q - 1)D$, $c = A + (r - 1)D$
$\left| {\begin{array}{*{20}{c}}{\,a\,\,\,}&{p\,\,\,}&{1\,}\\{\,b\,\,\,}&{q\,\,\,}&1\\{\,c\,\,\,}&{r\,\,\,}&1\end{array}} \right| = \left| {\begin{array}{*{20}{c}}{\,A + (p - 1)D\,\,\,}&{p\,\,\,}&{1\,}\\{A + (q - 1)D\,\,\,}&{q\,\,\,}&1\\{A + (r - 1)D\,\,\,}&{r\,\,\,}&1\end{array}} \right|$
Operate ${C_1} \to {C_1} - D{C_2} + D{C_3}$
$ = \,\left| {\begin{array}{*{20}{c}}{\,A\,\,}&{p\,\,}&{1\,}\\{\,A\,\,}&{q\,\,}&1\\{\,A\,\,}&{r\,\,}&1\end{array}} \right| = A\left| {\begin{array}{*{20}{c}}{\,\,1}&{\,\,p}&{\,\,1\,}\\{\,1}&q&1\\{\,1}&r&1\end{array}} \right| = 0$.