Only answers A, C, and D are in standard form, so we can rule B out immediately.
If you throw (5,1) into A, you get:
5(5) - 2(1) = 23
25 - 2 = 23
This checks, so A *might* be the answer.
If you throw (5,1) into C, you get:
2(5)-3(1) = 9
10 - 3 ≠ 9
This doesn't check, so it's not C.
If you throw (5,1) into D, you get:
5(5)+2(1) = 23
25 + 2 ≠ 23
This doesn't check, so it's not D.
The only standard form equation that (5,1) is a solution for is A.
Your answer is A.