Which of the following best describes a ligand?

A ligand is best described as a molecule that donates electrons to a central atom, typically a metal ion, to form a coordination complex. In this context, a ligand acts as a Lewis base, sharing a pair of electrons with the metal, which is considered a Lewis acid. This electron donation enables the formation of various chemical structures and plays a critical role in numerous biochemical and industrial processes, such as catalysis, drug design, and the function of metalloproteins.

The other choices provided do not accurately capture the definition of a ligand. A type of ionic compound refers to compounds formed through ionic bonds, which does not specifically involve electron donation to a central atom. A solid formed by a mixture of elements describes alloys or composite materials, which is unrelated to the interaction between ligands and metal ions. Lastly, a solvent used in chemical reactions refers to a medium that dissolves solutes, rather than a molecule that participates in the donation of electrons. Thus, recognizing that ligands specifically serve to coordinate with metal centers through electron donation clarifies their role in coordination chemistry.