b
Complex \(\left[{Co}({en})_{3}\right] {Cl}_{2}\) is most stable complexamong the given complex compounds because more number of chelate rings are present in thiscomplex as compare to others.

\((1)\) \(\left[{Co}({en})\left({NH}_{3}\right)_{4}\right] {Cl}_{2}\) \(\quad\quad\quad1\) chelate ring

\((2)\) \(\left[{Co}({en})_{3}\right] {Cl}_{2}\) \(\quad\quad\quad\quad\quad\quad3\) chelate ring

\((3)\) \(\left[{Co}({en})_{2}\left({NH}_{3}\right)_{2}\right] {Cl}_{2}\) \(\quad\quad\quad2\) chelate ring

\((4)\) \(\left[{Co}\left({NH}_{3}\right)_{6}\right] {Cl}_{2} \quad\quad\quad\quad\quad0\) chelate ring