(c)
Since, $b = \frac{a + c}{2}$ ...(i)
Now, discriminant, $D = B^{2} - 4AC$
$= 4b^{2} - 4ac$
$= 4\left( \frac{a + c}{2} \right)^{2} - 4ac$ [from Eq. (i)]
$= (a - c)^{2} \geq 0$
$\therefore$ Roots of the given equation are rational and distinct