3. Hydrochloric acid is used to clean metals. The acid reacts with the oxide layer on the surface of the metal, forming a salt and water.
Which word describes the metal oxide?
A alloy
B base
C element
D indicator

The answer to your question is option b, a base. This is because the question clearly tells you that one reactant is an acid. Then, it tells you that an acid plus something gives you a salt and water, as products

From background knowledge, we know that a base plus acid gives you a salt and water. The hydroxide ions from the base neutralise the hydrogen ions from the acid, which gives you water ( as it has a pH of 7).

Therefore, your answer is b, a base