Final answer:
Water is the inorganic substance present in the greatest quantity inside animal cells, making up about 70-90% of the cell's weight.
Step-by-step explanation:
The inorganic substance present in the greatest quantity inside animal cells is water. When considering the options provided - calcium, sodium, iron, and water - water is the most abundant molecule in cells, generally making up about 70-90% of the cell's weight. None of the other substances listed, such as calcium, sodium, or iron, approach the concentration levels of water within the cell.
While elements like calcium and sodium are vital to cellular function, they are present in much lower quantities. Calcium often has a role in signaling, while sodium is critical for functions such as nerve impulse transmission and cellular homeostasis. However, it's also important to note that sodium ions are primarily found at a higher concentration in the extracellular fluid, rather than inside the cells.