As shown in Fig., the height attained by the bob when the string subtends an angle $\mathrm{\alpha }$ with the vertical is

$\mathrm{h}=\mathrm{l}-\mathrm{lcos}\mathrm{\alpha }=\mathrm{l}(1-\cos \mathrm{\alpha })$

Its potential energy at the highest point A = mgh, Where m is the mass of the bob. Let v be the speed of the bob when it passes through O. Its kinetic energy at $\mathrm{O}=\frac{1}{2}{\mathrm{mv}}^2$. From the principle of conservation of energy, we have

$\frac{1}{2}{\mathrm{mv}}^2=\mathrm{mgh}$

$\mathrm{v}=\sqrt{2\mathrm{gh}}=\sqrt{2\mathrm{gl}(1-\mathrm{cos\alpha })}$

Hence the correct choice is (c).