Which of the following is a function of water in biological systems?

Water plays a crucial role in biological systems, and one of its key functions is acting as an excellent solvent. This characteristic is primarily due to the polar nature of water molecules, which allows them to interact with various solutes, facilitating chemical reactions within cells and the transport of nutrients and waste products.

In biological contexts, the ability of water to dissolve a wide range of substances is essential for processes like metabolic reactions, where enzymes and substrates must interact. This solvent property helps maintain homeostasis, as it allows for the effective distribution of ions, proteins, and other molecules necessary for life.

Other options, while significant in their own right, do not accurately describe the primary functions of water within biological systems. For instance, water does not provide energy to cells directly, nor does it increase cell rigidity or store genetic information. These functions are attributed to biomolecules such as carbohydrates, structural proteins, and nucleic acids, respectively, not water itself. Thus, the role of water as a solvent is fundamental to many life processes, making it the correct choice.