Question

The iron-containing protein found in red blood cells isO A) hemoglobin.O B) white blood cells,O C) platelets.O D) plasma.

Answer 1

I assume you are not familiar with this topic. I suggest you read more about this protein from online books or ur own book if you have. 
White blood cell = its a cell just like the name suggests, in fact, more like its a family of cell types that are responsible for immunity and some other functions (it doesnt look like you need to know the details on other functions so just stick with immunity) 
Platelets = these are not really cells, but you can say they are sorta like bits of cells, they contain important protein(s) for blood clotting and surely they themselves play important role in blood clotting which prevents us from bleeding to death when we get a cut for example. 
Plasma = this is part of blood, excluding red blood cells and white blood cells, its basically fluid with nutrients, ions and most importantly blood proteins, such as carrier ones like Albumin. 

The only one left is Hemoglobin,
heme is the iron part (which is not really iron alone but thats the only one u need to know) 
and globin is the protein part 
these two make up hemoglobin 
so this complex resides inside red blood cells, which flow in our vessels to every part of our body supplying them with nutrients and oxygen taken from our capillary system of lungs.