(d)
Since, $E_{1},\ E_{2}$are mutually exclusive events, then
$P\left( E_{1} \cap E_{2} \right) = 0$
Now, $P\left( E_{1} \cup E_{2} \right) = P\left( E_{1} \right) + P\left( E_{2} \right) - P{(E}_{1\ } \cap E_{2})$
$\Rightarrow 0.6 = P\left( E_{1} \right) + 1 - P\left( \overline{E_{2}} \right) - 0$
$\Rightarrow 0.6 = P\left( E_{1} \right) + 0.4$
$\Rightarrow P\left( E_{1} \right) = 0.2$