To dilute a sample, which of the following should be added?

To dilute a sample, the appropriate addition is a diluent. A diluent is specifically designed to reduce the concentration of a solution while maintaining the integrity of the sample. In laboratory practices, a diluent can be a liquid such as saline or other appropriate solutions that do not interfere with the analysis that is to be performed.

In the context of dilution, water can be used as a diluent in some cases, but it is not the only option, and using a specific diluent is often preferable to ensure accurate results and compatibility with the sample. Heparin is an anticoagulant that is used primarily for preventing clotting in blood samples rather than acting as a diluent, and sodium citrate serves a similar purpose in blood sample collection as an anticoagulant but does not dilute a sample. Thus, choosing a designated diluent is the most appropriate method for achieving the desired dilution without compromising the sample's integrity.