Answer
(a) $e^x$ is not a solution.
(b) $\sin x$ is a solution.
(c) $\cos x$ is a solution.
(d) $\sin x-\cos x$ is a solution.
Work Step by Step
Consider the linear differential equation $y^{\prime \prime}+y=0$
(a) $e^x$ is not a solution.
(b) $\sin x$ is a solution.
(c) $\cos x$ is a solution.
(d) $\sin x-\cos x$ is a solution.
