The correct answer is : something
A pronoun antecedent is a word that comes before a pronoun to which the pronoun refers. The word 'antecedent' itself means something that precedes something else. A pronoun is a word used to stand for or take place of a noun.
In the above sentence, there are three pronouns: something, it and their.
It is a subjective personal pronoun and it refers back to the word fort in the sentence ( it = the front would fall ).
Their is a possessive personal pronoun and it refers back to the words the French in the sentence ( their = in the hands of the French ).
Something is an indefinite pronoun and it does not have a word to refer back to in the sentence. It does not a specific antecedent.
