On a test, Marcus is asked to simplify √128 Here is his work:

√128 = √16 · √8 = 4√8

Which statement best describes Marcus' answer?

He is incorrect because 16 times 8 is not 128.
He is incorrect because the 4 and the 8 should be switched around in his answer.
He is correct.
He is incorrect because he did not use the largest perfect square.