Which of the following is a second order differential equation?

The correct answer is Option (2) → $y' y'' + y = \sin x$ ##

The second order differential equation is $y' y'' + y = \sin x$.

$\Rightarrow \left( \frac{dy}{dx} \right) \left( \frac{d^2y}{dx^2} \right) + y = \sin x$