Something that precedes something else is referred to as antecedent. Therefore, pronoun-antecedent refers to the word which precedes the pronoun, or to which the pronouns refer. The pronouns are the words which are used in place of a noun. Hence, the pronoun has to agree in number that is if the antecedent is singular then the pronoun used must be singular to take its place.
Hence, in the above sentence, the antecedent of the pronoun "their" is d. representatives. The pronoun 'their' agrees with the number of plural with the word representatives.