If the equation $a|z|^2 + \overline{\bar{\alpha}z + \alpha\bar{z}} + d = 0$ represents a circle where $a, d$ are real constants then which of the following condition is correct?
\( |\alpha|^2 - ad > 0 \) and \( a \in \mathbb{R} - \{0\} \)
\( |\alpha|^2 - ad \neq 0 \)
\( \alpha = 0, \; a, d \in \mathbb{R}^+ \)
\( |\alpha|^2 - ad \ge 0 \) and \( a \in \mathbb{R} \)