Read the sentence.
Thomas Jefferson and John Adams had an up-and-down, long-term friendship, which he wrote about in letters to friends.
Which sentence corrects the vague or ambiguous pronoun he?
Thomas Jefferson and John Adams had an up-and-down, long-term friendship, which was written by them to friends.
Thomas Jefferson and John Adams had an up-and-down, long-term friendship, which he wrote about to their friends.
Thomas Jefferson and John Adams had an up-and-down, long-term friendship, which was written about in letters to their friends.
Thomas Jefferson and John Adams had an up-and-down, long-term friendship, which Jefferson wrote about in letters to friends.