(a) $S \rightarrow SS\ |\ a$
This grammar is ambiguous. Take the string $aaa$ for example. To generate this string we can either have:
$S \rightarrow SS \rightarrow Sa \rightarrow SSa \rightarrow Saa \rightarrow aaa$
or
$S \rightarrow SS \rightarrow aS \rightarrow aSS \rightarrow aaS \rightarrow aaa$.
(b) $S \rightarrow 0S1\ |\ 01S\ |\ \epsilon$
This grammar is also ambiguous. The string $01$ have two different derivation trees.
$S \rightarrow 0S1 \rightarrow 01$
and
$S \rightarrow 01S \rightarrow 01$
(c) This statement is TRUE. Here's how we can generate the string $yxxyy$ from the given grammar :-
$S \rightarrow U \rightarrow yT \rightarrow yxSy \rightarrow yxTy \rightarrow yxxyy$
So all the three statements are TRUE.
Hence answer is (4).
P.S. I have not shown the derivation trees, but you can get the idea from the derivations.