Ended up using the below method, which isn't the fastest, but did get me to the right answer on my first attempt.
1. Using answer choices, I eliminated a) 1 because there are no positive integers that can sum to obtain 1 (keyword being positive and not non-negative, which could have added 0 into the mix as a possibility).
2. Starting with d) 3, I listed the possible positive addends that would sum to the answer choices and selected d) because it had the fewest (i.e., fastest to calculate). 1 + 2 are the only positive addends, so x could be 12 or 21, both of which aren't divisible by 5.
3. With answer c) 8, I listed: 1+7 (17 and 71 not divisible by 5), 2+6 (26 and 62 not divisible by 5), 3+5. Knowing 35 is divisible by 5, I started there and divided by the divisor 4 to see if the remainder was 1 (it's not, R = 3). Knowing 53 could not work (not divisible by 5), I moved to answer b.
4. b) 7 is possible from 1+6 (16 and 61 not divisible by 5), 2+5 (started with 25).
25/4 = 6 R 1 --> works
25/3 = 8 R 1 --> works
25/2 = 12 R 1 --> works
Answer b) 7.