Diluent
A diluent is simply the liquid added to a powder or a concentrate to dissolve or dilute it.
The word describes the role the liquid is playing rather than what it is. Bacteriostatic water is a diluent; so is plain sterile water; so is saline in other contexts.
Which diluent is appropriate is specified by a product's own labelling, and is not a preference. Recording which container of diluent was used, and when, is what makes a record traceable later.
What it is not
A diluent is not automatically bacteriostatic water. The two words are often used as though interchangeable, and they are not — one is a category, the other a specific product.