Final answer:
The correct relative pronoun is 'who'.
Step-by-step explanation:
The correct relative pronoun to complete the sentence is who.
Relative pronouns are used to connect a dependent clause to a main clause and refer to a noun that came before. In this sentence, the relative pronoun refers to the noun 'alchemists'.
An example of the completed sentence is: 'Alchemists, who wanted to transform base metals into gold, were also among the first people to conduct laboratory experiments.'