(d)
The given equation is
$\overrightarrow{\mathbf{r}^{\mathbf{2}}} - 2\overrightarrow{\mathbf{r}} \bullet \overrightarrow{\mathbf{c}} + h = 0,\ \left| \overrightarrow{\mathbf{c}} \right| > \sqrt{h}$
This is the equation of sphere in diameter form.
$ie,\ \left( \overrightarrow{\mathbf{r}} - \overrightarrow{\mathbf{a}} \right) \bullet \left( \overrightarrow{\mathbf{r}} - \overrightarrow{\mathbf{b}} \right) = 0$