Answer:
Step C error with expression sqrt(16)
Explanation:
so in step B look closely at the very first square root...it says

now in step C look again at the first square root...it says
, without any other obvious numerical arrangements to account for this error
essentially, sqrt(16) is 4, and sqrt(4) is 2, and they took the square root of 16 incorrectly in step C. It should've been just 4, not sqrt(4).