What is $Z$ in the following reactions?

Which of the following statements is correct?

$B_4C$ (boron carbide) is used except:

For a $p$-block element,its $3d, 3s, 3p$ and $4s$ orbitals are completely filled and the differentiating electron goes to the $4p$ orbital. The element should have its atomic number in the range:

In the structure of diborane $(B_2H_6)$,the number of $2$-centre-$2$-electron $(2c-2e)$ bonds is $X$ and the number of $3$-centre-$2$-electron $(3c-2e)$ bonds is $Y$. The value of $(X+Y)$ is:

In which of the following,a salt of the type $KMO_2$ is obtained?