Which of the following has a $4:2$ coordination number structure?

How many "nearest" and "next nearest" neighbours,respectively,does potassium have in a $bcc$ lattice?

What is the coordination number in a calcium fluoride $(CaF_2)$ type structure?

If the radius ratio limit is $0.414 - 0.732$,what will be the coordination number?

Coordination number of $Na^{+}$ ion in rock salt is

$A$ crystal is formed by two elements $X$ and $Y$ in a cubic structure. $X$ atoms are at the corners of a cube,while $Y$ atoms are at the face centers. The formula of the compound will be: