Answer:
Roger Bannister, Gunder Hägg
Step-by-step explanation:
Antecedent is a noun that is replaced by a pronoun in order to avoid repetition. For examle:
When he saw George, he yelled at him.
Him is a pronoun that replaces George, so George is antecedent for this pronoun.
Now, one should always write pronouns in a way that they have only one clear antecedent. When this is not the case we say there is a vague antecedent.
In the first part of the text we are introduced to two track runners, Bannister and Hägg. Since the next sentence starts with the pronoun "he" it is unclear to whom it refers.
The pronoun is vague and the possible antecedents are the names of the two racers.