The homogeneous equation
y'' + y = 0
has characteristic equation
r² + 1 = 0
with roots at r = ± √(-1) = ± i, and so the characteristic solution is
y = C₁ cos(x) + C₂ sin(x)
Using the method of undetermined coefficients, suppose
y = a x cos(x) + b x sin(x)
(we would normally consider y = a cos(x) + b sin(x), but cos(x) and sin(x) are already part of the characteristic solution)
The derivatives are
y' = a cos(x) - a x sin(x) + b sin(x) + b x cos(x)
y' = (b - a x) sin(x) + (a + b x) cos(x)
and
y'' = -a sin(x) + (b - a x) cos(x) + b cos(x) - (a + b x) sin(x)
y'' = (-2a - b x) sin(x) + (2b - a x) cos(x)
Then
y'' + y = (-2a - b x) sin(x) + (2b - a x) cos(x) + a x cos(x) + b x sin(x)
7 sin(x) = -2a sin(x) + 2b cos(x)
and it follows that a = -7/2 and b = 0.
So, the general solution to the DE is
y = C₁ cos(x) + C₂ sin(x) - 7/2 x cos(x)
which means the only valid solutions from the given choices are
y = 7 sin(x)
y = 7 cos(x)
