Q. The grammar whose productions are
<stmt> → if id then <stmt>
<stmt> → if id then <stmt> else <stmt>
<stmt> → id := id
is ambiguous because
a) the sentence if a then if b then c:= d has two parse trees
b) the left most and right most derivations of the sentence if a then if b then c:= d give rise to different parse trees
c) the sentence if a then if b then c:= d else c:= f has more than two parse trees
d) the sentence if a then if b then c:= d else c:= f has two parse trees