Ion which scatters X-ray most is -
The scattering is proportional to the outer shell electrons available in the medium.

i) Calcium has 2 electrons in the outer shell. But the option is calcium ion (Ca++), that means, it has donated both electrons, i.e. Ca++ ion has no outer shell electron.
ii) Mercury has 2 electrons in the outer shell.
iii) Lead has 4 electrons in outer shell
iv) H+ ion has no electron in outer shell as it has donated the only electron of its outer shell.

Amongst the given options lead has maximum electrons in its outer shell. Therefore, the lead will cause maximum scattering of X-rays.
Ca++
Hg
Pb
H+