Find the point of intersection of normals at any two points on the parabola $y^2=4ax$

Question

$\begin{array}{1 1}(a)\;k=-at_1t_2(t_1+t_2)\\(b)\;k=-at_1t_2(t_1-t_2)\\(c)\;k=-at_1t_2(t_2-t_1)\\(d)\;\text{None of these}\end{array}$