y = a sin x + b cos x is the solution of differential equation :

$\frac{d y}{d x}=a \cos x-b \sin x \Rightarrow \frac{d^2 y}{d x^2}=-a \sin x-b \cos x=-y$

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

Hence (1) is the correct answer.