The correct option is A Iridium
Down the group in d-block size doesn't increase as much because of poor shielding of f-orbitals. So, volume doesn't increase much but mass increases, hence, density increases.
Across the group in d-series, density increase due to decrease in radii and increase in atomic mass.
Hence, Iridium has the highest density among others.
Hence, option (a) is correct.