The transition elements and their compounds are known for their catalytic activity. Which of the following is the reason for their catalytic activity?

$2{\mathrm{CuX}}_2\stackrel{\text{ Room temperature }}{\to }2\mathrm{CuX}+{\mathrm{X}}_2$

What is ${\mathrm{X}}^{-}$?

Which of the following statements are correct about $\mathrm{Zn}, \mathrm{Cd}$and $\mathrm{Hg}$?

a. They exhibit high enthalpies of atomization as the $\mathrm{d}-$sub-shell is fully filled.
b. $\mathrm{Zn}$and $\mathrm{Cd}$do not show variable oxidation states while $\mathrm{Hg}$does show.
c. Compounds of $\mathrm{Zn}, \mathrm{Cd}$ and $\mathrm{Hg}$are paramagnetic.
d. $\mathrm{Zn}, \mathrm{Cd}$and $\mathrm{Hg}$are soft metals.

The incorrect statement about ${\mathrm{Cr}}^{2+}$ and ${\mathrm{Mn}}^{3+}$ is/are (Atomic number $\mathrm{Cr}=24, \mathrm{Mn}=25$).