For standardizing $NaOH$ solution,which of the following is used as a primary standard?

In an acid-base titration,$0.1 \ M \ HCl$ solution was added to the $NaOH$ solution of unknown strength. Which of the following correctly shows the change of $pH$ of the titration mixture in this experiment?

An aqueous solution of $6.3 \ g$ of oxalic acid dihydrate is made up to $250 \ mL$. The volume of $0.1 \ N \ NaOH$ required to completely neutralise $10 \ mL$ of this solution is.......$mL$

In the titration of $I_{2(aq)}$ by $S_2O_3^{2-}{_{\text{(aq)}}}$ using the starch indicator,the end point is indicated by:

The strength of an aqueous solution of $I_2$ can be determined by titrating the solution with a standard solution of:

$3.92 \, g$ of ferrous ammonium sulphate crystals are dissolved in $100 \, mL$ of water. $20 \, mL$ of this solution requires $18 \, mL$ of $KMnO_4$ during titration for complete oxidation. The weight of $KMnO_4$ present in one litre of the solution is ............. $g$.