Answer
(a) iron
Explanation
Subgroup is a smaller collection of elements from a group that is it self a group is called a subgroup. These all metals belongs to group of transition metals. But there are many subgroups in transition metals.
Cobalt, Iridium and Rhodium belongs to subgroup cobalt because these all fall in same Group ( Group 9A) but Fe belongs to group 8A. All group 9A elements are relatively rare in the earth's crust, with the most abundant, cobalt, only accounting for 0.0029% of the Earth's crust. Rhodium and iridium are two of the rarest naturally occurring elements in the earth, only found in platinum ores. Inversely Fe has abundance in nature.
