Answer
{e, ab}
Work Step by Step
using set notation to derive the language defined by the e | ab regular expression.
Assuming $\Sigma$ = {a, b, c}.
$L(e | ab) = L(e) ∪ L(ab) = {e} ∪ L(a)L(b)$
$= ({e}) ∪ {xy | x ∈ L(a) and y ∈ L(b)}$
$= ({e}) ∪ {xy | x ∈ {a} and y ∈ {b}}$
$= ({e}) ∪ {ab} = {e, ab}$
