The correct answer is option 'A', potassium.

Water content of the blood is tightly regulated by the body in order to maintain homeostasis and ensure proper bodily functions. The control of water content is primarily achieved through a complex mechanism involving several organs, hormones, and ions.

Potassium, an essential mineral and electrolyte, plays a crucial role in the control of water content in the blood. Here is a detailed explanation of how potassium is involved in this process:

1. Regulation of osmolarity:
- Osmolarity refers to the concentration of solutes in a solution. The body maintains a delicate balance of osmolarity in the blood to prevent water from moving in or out of the cells excessively.
- Potassium ions (K+) are involved in the regulation of osmolarity by influencing the movement of water across cell membranes.
- When the concentration of potassium ions is higher outside the cell compared to inside, water moves out of the cells to balance the osmolarity. Conversely, when the concentration is higher inside the cell, water moves into the cells.

2. Role in maintaining fluid balance:
- The kidneys play a vital role in maintaining fluid balance by regulating the amount of water reabsorbed and excreted in urine.
- Potassium acts as an osmotic agent in the kidneys, influencing the reabsorption and excretion of water.
- It helps to regulate the concentration of solutes in the filtrate, which determines the amount of water that is reabsorbed back into the bloodstream.

3. Interaction with aldosterone:
- Aldosterone is a hormone produced by the adrenal glands that helps regulate fluid balance by increasing the reabsorption of sodium ions and water in the kidneys.
- Potassium levels in the blood influence the release of aldosterone. When potassium levels are low, aldosterone is released, which enhances sodium reabsorption and water retention, leading to an increase in blood volume.

In conclusion, potassium is involved in the control of water content in the blood through its role in regulating osmolarity, maintaining fluid balance in the kidneys, and interacting with aldosterone. It helps to ensure that the water levels in the blood are adequately regulated, contributing to overall homeostasis in the body.