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

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

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

The general equation of non-vertical lines in a plane is $a x+b y=1, b \neq 0$ $\Rightarrow a+b \frac{d y}{d x}=0 \Rightarrow \frac{d y}{d x}=-\frac{a}{b} \Rightarrow \frac{d^2 y}{d x^2}=0$ Hence (3) is the correct answer.