Final answer:
The best relative pronoun to complete the sentence is 'who'. Relative pronouns are used to provide additional information about something mentioned in the sentence.
Step-by-step explanation:
To complete the sentence, we should use the relative pronoun 'who'. So, the entire sentence could be: 'At the craft fair, two of the artists who we've admired for years were selling ceramic bowls painted with Aztec motifs.' Relative pronouns, such as 'who', are utilized to provide extra information about something referred to in the sentence.
Learn more about Relative Pronouns