Select each function y=f(x) that is a solution of the given differential equation. y''+64y=0

Select all solutions of y''+64y=0

a. y= 3sin (3x)

b. y= c_1 cos(8x)+c_2 sin (8x)

c. y= 3 cos (8x) + 4 cos (8x)

d. y= c_1 sin (4x) + c_2 cos (4x)