For two sets \(A \cup B = A\) iff
(a) \(B\subseteq A\)
(b) \(A\subseteq B\)
(c) \(A\ne B\)
(d) \(A=B\)
Solution
If
\[ A\cup B=A \]
then every element of \(B\) is already present in \(A\).
Therefore,
\[ B\subseteq A \]
Answer
\[ \boxed{B\subseteq A} \]
Correct option: (a)