Answer
(a), (b), and (d) are separable.
Work Step by Step
(a) is separable, $$ y'=f(y)g(x); \quad f(y)=9y^2, \quad g(x)=1/x.$$
(b) is separable, $$ y'=f(y)g(x); \quad f(y)=e^{3y}, \quad g(x)=\sin x/\sqrt{4-x^2}.$$
(c) is not separable (notice the addition sign).
(d) is separable, $$ y'=f(y)g(x); \quad f(y)=9-y^2, \quad g(x)=1.$$
