Final answer:
The best relative pronoun to complete the sentence is 'who.'
Step-by-step explanation:
The best relative pronoun to complete the sentence is who.
An example of the corrected sentence would be:
Muriel Siebert was a college dropout who broke barriers in the male-dominated world of finance in the 1960s and 1970s, becoming the first woman to own a seat on the New York Stock Exchange.
Learn more about relative pronouns in English grammar