Question: For two sets $A \cup B=A$ iff
(a) $B \subseteq A$
(b) $A \subseteq B$
(c) $A \neq B$
(d) $A=B$
Solution:
(a) $B \subseteq A$
The union of two sets is a set of all those elements that belong to A or to B or to both A and B.
If $A \cup B=A$, then $B \subseteq A$
