Harry knew he was supposed to save Ron because throughout the books, he hinted many times that he preferred Ron as a friend over Hermione. For example, in GoF:
"Harry liked Hermione very much, but she just wasn't the same as Ron. There was much less laughter and a lot more hanging around in the library when Hermione was your best friend." -Harry Potter and the Goblet of Fire, Chapter 19: The Hungarian Horntail
To sum this up, in terms of friends, Harry chose Ron over Hermione. He also knew that Viktor had taken Hermione to the Yule Ball.
"His eyes fell instead on the girl next to Krum... It was Hermione." -Harry Potter and the Goblet of Fire, Chapter 23: The Yule Ball
And Harry knew that Cedric took Cho to the Yule Ball(giving that he had also asked Cho out).
"'Oh—Cedric,' she said. 'Cedric Diggory.'" -Harry Potter and the Goblet of Fire, Chapter 22: The Unexpected Task
And Harry did not know Gabrielle before the task.
"Ron was tied between Hermione and Cho Chang. There was also a girl who looked no older than eight..." Harry Potter and the Goblet of Fire, Chapter 26: The Second Task
Every other hostage was narrated by name except for Gabrielle.
So, Harry knew to choose Ron, and he would have immediately chosen Ron, based on who the contestants are and the connections of them to the other Triwizard champions.