This is in Greg’s explanation video for Algebra Fundamentals Quiz #1. (ignore the 1’s and -1’s, that was an example of how choosing numbers worked.) Greg described how simplification only works here (i.e. reducing everything to x^3) if you know that option two isn’t x^3, it’s |x^3| So how is option two abs value but the others (including the one in the question) aren’t?

Hint: the given expression can be negative. Can \sqrt{x^6} ever be negative?

Think about the powers and negative values of x.