Which element from following is radioactive?

Which among the following statements about $$\left[\mathrm{Ni}(\mathrm{CN})_4\right]^{2-}$$ is $$\underline{\text { NOT }}$$ true?

Formation of $$\mathrm{NO}_{2(\mathrm{g})}$$ from $$\mathrm{N}_{2(\mathrm{g})}$$ and $$\mathrm{O}_{2(\mathrm{g})}$$ is an endothermic process. Which of the following is true for this reaction?

Identify reducing agent in following reaction

$$\mathrm{H}_2 \mathrm{O}_{2(\mathrm{ag})}+\mathrm{ClO}_{4(\mathrm{aq})}^{-} \rightarrow \mathrm{ClO}_{2(\mathrm{aq})}^{-}+\mathrm{O}_{2(\mathrm{g})}$$