If two sets are disjoint, then what region represents the union of their complements in a Venn diagram?

If 2 sets are disjoint means they have not any common element.

Draw a rectangle & inscribe inside this rectangle 2 circles that do not intersect.

cercle₁ ∩ cercle₂ = Ф.

Their complement would be the part of the rectangle - the 2 circles