1. The problem asks for the meaning of the notation $A \setminus B$. 2. In set theory, $A \setminus B$ denotes the set of elements that are in $A$ but not in $B$. 3. This is also called the set difference of $A$ and $B$. 4. Therefore, the correct interpretation is option (a): Elements in $A$ but not in $B$. Final answer: (a) Elements in A but not in B.