Which ion is indicated as having a 2- charge and is commonly encountered in solutions?

The ion with a 2- charge that is commonly encountered in solutions is oxalate. This ion is represented as C2O4^2-. It typically forms when oxalic acid (H2C2O4) dissociates in solution, releasing two protons. Oxalate is often found in various natural substances and can be present in biological systems as well.

Other options, while relevant ions, do not carry a 2- charge. Nitrate (NO3^-) carries a 1- charge, thiosulfate (S2O3^2-) also has a 2- charge, but it is less commonly encountered, and sulfite (SO3^2-) has a 2- charge as well, but it is notably different from oxalate in both structure and common occurrence in various applications.

Thus, oxalate stands out as the most typical representation of a 2- charge ion that one encounters frequently in aqueous solutions.