Solution of differential equation xdy – ydx = 0 represents

$x d y-y dx=0 \Rightarrow \frac{d y}{y}-\frac{d x}{x}=0 $

⇒ log y log x = log c

$\Rightarrow \log \left(\frac{y}{x}\right)=\log c$

$\Rightarrow \frac{y}{x}=c$ or y = cx which is a straight line.

Hence (2) is the correct answer.