If the locus of \( z\in \mathbb{C} \), such that \( Re\!\left(\dfrac{z-1}{2z+i}\right)+Re\!\left(\dfrac{\overline{z}-1}{2\overline z-i}\right)=2 \), is a circle of radius \( r \) and centre \( (a,b) \), then \( \dfrac{15ab}{r^2} \) is equal to:
\(12\)
\(16\)
\(18\)
\(24\)
