Which of the following contains maximum number of molecules?

What is the atomic number of an element having $ns^1$ electronic configuration and belonging to 3d transition series?

The electronic configurations of elements are as-
$A = 1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2$
$B = 1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^5$
Which of the following is formula of ionic compound that could be formed between these two elements A and B?

Which of the following pairs represents isobars?