Obstacles make us stronger is a theme. A theme is the life lesson the author tries to teach his or her readers. A topic is like what the story is about. For example, a movie can have different topics. An example of a movie topic would be action, comedy, or romance. A theme could be like the tortoise and hare, slow and steady wins the race. A book or movie could be about bravery and being afraid, or struggle and endurance. The last answer, Winning became Sam's goal, is not a topic nor a theme.
And to be honest, obstacles make us stronger sounds like that one song what doesn't kill you makes you stronger.
