The universal set should be $A \bigcup B$.

Explanation:

We know, $A \Delta B =\left ( A \bigcup B \right ) - \left ( A \bigcap B \right )$

$A \Delta B = \left ( A \bigcap B \right )'$ = Universal Set - $\left ( A \bigcap B \right )$

$\left ( A \bigcup B \right ) - \left ( A \bigcap B \right )$ = Universal Set - $\left ( A \bigcap B \right )$

=> Universal Set = $\left ( A \bigcup B \right ) $