Which of the following is the correct order in the spectrochemical series of ligands?

Which of the following complex ions is the most stable?

If an excess of $NH_4OH$ is added to a $CuSO_4$ solution,it forms a deep blue coloured complex which is:

Assertion : $[Al(H_2O)_6]^{3+}$ is a stronger acid than $[Mg(H_2O)_6]^{2+}$.
Reason : Size of $[Al(H_2O)_6]^{3+}$ is smaller than $[Mg(H_2O)_6]^{2+}$ and possesses more effective nuclear charge.

$5.33 \text{ g}$ of $CrCl_3 \cdot 6H_2O$,which is a $1:3$ electrolyte,is dissolved in water and passed through a cation exchanger. The chloride ions in the eluted solution,on treatment with $AgNO_3$,result in $8.61 \text{ g}$ of $AgCl$. The ratio of moles of complex reacted and moles of $AgCl$ formed is . . . . . . $\times 10^{-2}$. (Nearest integer) [Molar mass in $\text{g mol}^{-1}$: $Cr = 52, Ag = 108, Cl = 35.5, H = 1, O = 16$]

Explain the chelating effect or chelation.