Find the set $A \cup \varnothing$. \[ \begin{array}{l} U=\{1,2,3,4,5,6\} \\ A=\{3,4,5,6\} \end{array} \] Select the correct choice below and, if necessary

Step 1 ：Given the universal set U = \{1, 2, 3, 4, 5, 6\} and set A = \{3, 4, 5, 6\}.

Step 2 ：The question is asking for the union of set A and the empty set.

Step 3 ：The union of any set with the empty set is the set itself.

Step 4 ：Therefore, the answer should be set A.

Step 5 ：Final Answer: The set A \(\cup\) \(\varnothing\) is \(\boxed{\{3, 4, 5, 6\}}\).