(i) Sodium hydroxide reacts with $Al$ corroding the metal and produces highly flammable hydrogen gas. Hence, it cannot be kept in $Al$ containers.
(ii) Silver is a highly unreactive metal so, it does not react with the oxygen of air easily. But silver jewellery tarnishes after some time (turns black) due to the formation of a thin sulphide layer on their surface by the action of hydrogen sulphide ($H2S$) gas present in air.
(iii) We cannot use an aqueous solution of sodium chloride to obtain sodium metal because if we electrolyse an aqueous solution of $NaCl$, then as soon as sodium metal is produced at cathode, it will react with water present in the aqueous solution to form sodium hydroxide. So, the electrolysis of aqueous solution of $NaCl$ will produce $NaOH$ instead of Na metal.
(iv) Al metal reacts with dilute $HCl$ slowly in the beginning due to the presence of a tough protective layer of aluminium oxide on its surface.