Final answer:
Iron is essential for a child's ability to run, speak, and write due to its role in oxygen transport and brain development.
Step-by-step explanation:
The correct answer is a) Iron. Iron is essential for the production of hemoglobin, which is responsible for carrying oxygen in the blood. Without adequate iron, a child may experience fatigue and have difficulty running due to decreased oxygen delivery to the muscles. Iron is also important for brain development and proper cognitive function, which are necessary for speaking and writing.