The differential equation that has $y=C_{1}cos3x+C_{2}sin3xas$ its general solution $is$:

$(a)y^{\text{'}\text{'}}-3y=0$

$(b)y^{\text{'}}-3y=cos3x$

$(c)y^{\text{'}\text{'}}+9y=0$

$(d)y^{\text{'}\text{'}}-9y=0$

$(e)$ None $of$ the above.