Identify the correct statement for following reaction.

$$3 \mathrm{Mg}+\mathrm{N}_2 \longrightarrow \mathrm{Mg}_3 \mathrm{~N}_2$$

Which of the following cannot be used as standard solution for determination of cell constant of conductivity cell?

Which from following reagents is used in the conversion of phenol to picric acid?

Which element from following has largest atomic size as compared to other three?