Which of the following is NOT a type of base?

A metal sulfate is not considered a type of base. Bases are substances that can accept protons (H⁺ ions) or donate electron pairs. They typically react with acids to form salts and water in a neutralization reaction.

Metal hydroxides, metal oxides, and metal carbonates are all bases. Metal hydroxides contain hydroxide ions (OH⁻) and react with acids to produce water and the corresponding metal salt. Metal oxides are compounds that contain a metal and oxygen, which can also act as bases by reacting with acids. Metal carbonates, containing carbonate ions (CO₃²⁻), can react with acids to release carbon dioxide and form a salt and water.

In contrast, metal sulfates do not exhibit basic properties like accepting protons or reacting with acids in a way that would classify them as bases, as they are typically neutral salts formed from the reaction of a metal oxide or hydroxide with sulfuric acid. Therefore, identifying metal sulfate as the option that does not fit the definition of a base is accurate.