Sorry, this is suuuuper late:
The accurate answer would be 26. If you read the passage, it clearly stated that "In 1905, at the age of twenty-six, Einstein published his now famous "Annus Mirabilis Papers". Also, I got a solid 100% on this. I'm not sure how the other person got 16 or if anyone read the passage, but anyways good luck on your future assignments.
Side note: Also, make sure to always put the answer options on your question or you might not get the correct answer.