What chemical solution is known to darken copper and brass?

The chemical solution that is known to darken copper and brass is potassium sulfide. This chemical reacts with the surface of these metals to form a blackened layer known as patina. Potassium sulfide is specifically used in processes where metal oxidation and the creation of a dark finish are desired.

The other options do not serve the same purpose. Vinegar, while acidic, can clean copper and help remove tarnish, but it does not produce a darkened patina. Sodium chloride (table salt) doesn't inherently react with copper or brass to darken them; it is more commonly used to clean or etch metal surfaces. Ammonium hydroxide (also known as ammonia) can clean metals or remove oxidation but does not darken them. Thus, potassium sulfide is the correct choice for achieving a darkened effect on copper and brass.