Which of the following metals is associated with hemoglobin?

1. The iron-containing molecule found in red blood cells is called hemoglobin. 
2. It is present in all vertebrates and aids in transporting oxygen. 
3. In fishes, the blood transports oxygen from the lungs or gills to the rest of the body. 
4. Mammals' red blood cells contain about 96 percent protein by weight and about 35 percent protein overall, including water.
5. In order to carry out aerobic respiration and other cell metabolism activities, hemoglobin releases oxygen. 
6. Hemoglobin has an oxygen-binding capacity and every 100 ml of blood from a healthy normal person has 12 to 20 grams of hemoglobin.

Hence, option B is the correct answer.