Which of the following is the most stable complex?

The aqueous solution of a transition metal salt changes colour from pink to blue when concentrated hydrochloric acid is added to it. The change in colour is due to:

Cobalt $(III)$ chloride forms several octahedral complexes with ammonia. Which of the following will not give a test for chloride ions with silver nitrate at $25 \ ^oC$?

Which of the following is not true for a ligand-metal complex?

$A$ complex containing $K^{+}$,$Pt(IV)$ and $Cl^{-}$ is $100\%$ ionised giving $i = 3$. Thus,the possible complex is

Which of the following is the most stable complex ion?