Given f (x) = sin x + cos x ... (i) Now. Replacing x by - x in (i) we get f (- x) = sin (- x) + cos ( - x) = - sin x + cos x [Since cos ( - θ) = cos θ, sin ( - θ) = - sin θ] Neither f ( - x) = f (x) nor f ( - x) = - f (x) ∴ f (x) is neither an even function nor an odd function Also, f (x) depends on x . So, f (x) is not a constant function. Hence, option 'D' is correct