A particle is rotated in a vertical circle by connecting it to a string of length l and keeping the other end of the string fixed. The minimum speed of the particle hen the string is horizontal for which the particle ill complete the circle is
$$\begin{gathered} \left(a\right) \sqrt{gl} \\ \left(b\right) \sqrt{2gl} \\ \left(c\right) \sqrt{3gl} \\ \left(d\right) \sqrt{5gl} \end{gathered}$$