Explanation

(b)
Given, $z\overline{z} + a\overline{z} + \overline{a}z + b = 0,\ \ \ b \in R$
On adding $a\overline{a}$ on both sides in the given equation, we get
$z\overline{z} + a\overline{z} + \overline{a}z + a\overline{a} + b = a\overline{a}$
$\Rightarrow \ \ \ \ \ (z - a)\left( \overline{z} + \overline{a} \right) = a\overline{a} - b$
$\Rightarrow \ \ \ \ |z + a|^{2} = |a|^{2} - b$
This equation will represent a circle, if
$|a|^{2} - b > 0\ \ \ \ \Rightarrow \ \ |a|^{2} > b$