The differential equation of all non-horizontal lines in a plane is :

$\frac{d y}{d x}=0$ $\frac{d x}{d y}=0$ $\frac{d^2 y}{d x^2}=0$ $\frac{d^2 x}{d y^2}=0$

$\frac{d^2 x}{d y^2}=0$

$a x=b y=0, a \neq 0 \Rightarrow \frac{d^2 x}{d y^2}=0$ Hence (4) is the correct answer.