Answer: Every person who completes the reading program gets a certificate of achievement.
Explanation:
A relative pronoun is a pronoun that is used to make reference to nouns that have been mentioned previously in a sentence. Examples include: that, who, whom, whose, which, etc.
The best relative pronoun for the sentence is “who”. This is because “who” as a relative pronoun is used to describe or make reference to a person or group of persons.
Therefore the sentence is best written as:
Every person who completes the reading program gets a certificate of achievement.
