In which of the following arrangement a metal could have least density

Which of the following transition metal ions has the lowest density?

Which of the following transition metal ions has the lowest density?

Which of the following have transition metal?

Which of the following have transition metal?

Which of the following alkaline earth metal has least melting point?

Which of the following crystalline arrangements will have at least one of the following equal to 90^(@) and at least two axial lengths same?

Gases have the least density.