In case d), barium boils at a lower temperature than strontium.
Boiling point is determined by the strength of the forces that hold the atoms together in the liquid phase. This problem has us investigating the trend in this force for elements in the same family on the periodic table.
In looking at metals, the trend is for this force to decrease as the size of the atoms increases, because while the charge on the kernel (the nucleus + all non-valence electrons) remains constant, the larger size of the kernel causes more distance between its centre and the mobile electrons that bond the atoms together.
Therefore, the boiling points of metals decrease as you move down a column.
In non-metals, the bonding is due to dispersion forces such as Van der Waals forces. These forces are due to momentary dipoles caused by instantaneous asymmetries in the distribution of the electrons. Even if you are not clear on that point, the pattern is that as the number of electrons increases (and the size of the atoms increase) the bonding forces increase.
So, for non-metals, the boiling points increase as you go down the column.