Which one of the following nitrates will leave behind a metal on strong heating?

The atomic numbers of vanadium (V), Chromium (Cr), manganese (Mn) and iron (Fe) are respectively 23, 24, 25 and 26. Which one of these may be expected to have the highest second ionization enthalpy?

For making good quality mirrors, plates of float glass are used. These are obtained by floating molten glass over a liquid metal which does not solidify before glass. The metal used can be :

The radius of La³⁺ (Atomic number of La = 57) is 1.06 Å. Which one of the following given values will be closest to the radius of Lu³⁺ (Atomic number of Lu = 71) ?