$u=u(x,y)=\sin (y+ax)-(y+ax)^2=>$

Question

$$(a)\;u_{xx}=a^2.u_{yy} \quad (b)\;u_{yy}=a^2u_{xx} \quad (c)\;u_{xx}=-a^2.u_{yy} \quad (d)\;u_{yy}=-a^2u_{xx}$$