Answer:
(A)
with
.
(B)
with 
(C)
with 
(D)
with
,
Step-by-step explanation
(A) We can see this as separation of variables or just a linear ODE of first grade, then
. With this answer we see that the set of solutions of the ODE form a vector space over, where vectors are of the form
with
real.
(B) Proceeding and the previous item, we obtain
. Which is not a vector space with the usual operations (this is because
), in other words, if you sum two solutions you don't obtain a solution.
(C) This is a linear ODE of second grade, then if we set
and we obtain the characteristic equation
and then the general solution is
with
, and as in the first items the set of solutions form a vector space.
(D) Using C, let be
we obtain that it must satisfies
and then the general solution is
with
, and as in (B) the set of solutions does not form a vector space (same reason! as in (B)).