Final answer:
The iron attached to heme is known as heme-bound iron, which is specifically present in hemoglobin and myoglobin and is fundamental to their oxygen-carrying function.
Step-by-step explanation:
The type of iron that is attached to heme in hemoglobin molecules is in the form of an iron ion in a specific oxidation state. Each heme group contains an iron ion that can change oxidation states as it binds and releases oxygen. This iron within the heme is neither ferritin, transferrin-bound iron, nor non-heme iron. Instead, it is referred to as heme-bound iron, which is the correct answer to the question. When not bound to heme, iron can be stored in ferritin or hemosiderin, or carried through the bloodstream bound to transferrin.