The correct option is A Surface tension
Osmosis is a process in which the water molecules move from lower solute concentration to higher solute concentration through a semipermeable membrane.
Viscosity is the property of liquid, that offers resistance to flow.
Protoplasm is a gel like substance present in all cells. It is a colloid of water and the disperse particles present in them. These particles are proteins, lipids, carbohydrates.
Surface tension is the property of the liquid or semisolids to acquire the least surface area possible. Due to surface tension, unicellular organisms keep spherical shape. Therefore, option A is correct.