Answer:
Option A.
Explanation:
If we have an expression as ( a+b ) = c
Then as per subtraction property of equality [(a+b) - b = c - b]
In this question step 6 is 18 + 5 - 5 = 21 - 5
If we compare step 6 with the statement above then we find ( 18 + 5 ) - 5 = 21 - 5
Therefore, option A subtraction property of equality will be the answer.