Understand the process of mixing drink powder into water.

When drink powder (a solid) is mixed into water (a liquid), it typically dissolves. Dissolution is a physical process where the solid breaks down into individual particles (ions or molecules) that become uniformly dispersed within the liquid.

• A. The particles of drink powder increase in size. This is incorrect. When a solid dissolves, its particles separate and become much smaller, often individual molecules or ions, which are too small to be seen. They do not increase in size.
• B. The particles of drink powder change into water particles. This is incorrect. Dissolution is a physical change, not a chemical reaction that transforms one substance into another. The drink powder particles retain their chemical identity, even when dissolved.
• C. The particles of drink powder spread throughout the water. This is correct. When drink powder dissolves, its particles disperse evenly throughout the water, creating a homogeneous mixture (a solution). This is why the drink changes color and flavor uniformly.
• D. The particles of drink powder change phase from solid to gas. This is incorrect. Changing directly from solid to gas is called sublimation. When drink powder is mixed with water, it dissolves into the liquid phase, it does not turn into a gas.

Option C accurately describes the process of dissolution, where the solute particles (drink powder) become uniformly distributed throughout the solvent (water).

The final answer is $\boxed{\text{C}}$