Which of the following describes an isotonic solution?

An isotonic solution is characterized by equal concentrations of solute particles on both sides of a cell membrane. This balance means that there is no net movement of water into or out of the cell, allowing the cell to maintain its normal shape and function.

In biological terms, an isotonic environment is crucial for maintaining cell homeostasis. When a cell is in an isotonic solution, the osmotic pressure is balanced, which prevents swelling or shrinking of the cell. Thus, stating that an isotonic solution maintains equal concentrations effectively captures the essence of this concept.

The other descriptions do not reflect the essential nature of isotonic solutions: fewer particles describe a hypotonic solution, higher concentration of particles pertains to a hypertonic solution, and a completely solid solution does not accurately represent the concept of tonicity, as this typically involves liquids where solute and solvent are present.