The slope of normal to the curve y=f(x) is m, then :

The correct answer is Option (4) → $m=-\frac{dx}{dy}$

Slope of tangent = $n=\frac{dy}{dx}$

we know that tangent ⊥ normal

$⇒mn=-1$

so $m=-\frac{1}{n}=-\frac{dx}{dy}$