b
$\begin{array}{l}\Delta=0=\left|\begin{array}{ccc}\alpha^2 & \alpha & 1 \\1 & 1 & 1 \\a & b & c\end{array}\right| \\\Rightarrow \alpha^2( c - b )-\alpha( c - a )+( b - a )=0\end{array}$
It is singular when $\alpha=1$
$\frac{(a-c)^2}{(b-a)(c-b)}+\frac{(b-a)^2}{(a-c)(c-b)}+\frac{(c-b)^2}{(a-c)(b-a)}$
$\frac{(a-b)^3+(b-c)^3+(c-a)^3}{(a-b)(b-c)(c-a)}$
$=3 \frac{(a-b)(b-c)(c-a)}{(a-b)(b-c)(c-a)}=3$