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

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.