Since n^2 -8 divided by 33 gives remainder r, I put it into the remainder formula to get n^2 - 8 =33x+r. Then I isolated n^2 by adding 8 to both sides.
n^2 = 33x + r + 8
Since 33 has factors of 3 and 11. we can use that 3 factor to filter the remainders of each term in the equation.
Squared numbers divided by 3 have a pattern of remainders of 0 and 1:
0^2 / 3 = 0/3, so remainder is 0
1^2 / 3 = 1/3, so remainder is 1
2^2 / 3 = 4/3, so remainder is 1
3^2 / 3 = 9/3, so remainder is 0
and so on.
33k will always be divisible by 3 and have a remainder of 0.
Lastly we will need the remainder for the term r+8 which we will find as we plug in the answer choices.
If we refer back to our equation n^2 = 33k + r + 8, we want the remainders of both sides to be equal. We can ignore 33k because the remainder is zero so we just need the remainder of r+8 to be either 0 or 1 to match n^2.
Now we plug in the remainder choices into r+8
0+8 = 8, 8/3 has a remainder of 2 so 0 is not a possible remainder.
15+8 = 23, 23/3 has a remainder of 2 so 15 is not a possible remainder
21+8 = 29, 29/3 has a remainder of 2 so 21 is not a possible remainder
The answer is E, none of these.