If x = 1, then 3*1 = 3 which when modded with 5, we get 3 as a remainder. In other words, 3/5 = 0 remainder 3. We don't use the quotient at all when it comes to modular arithmetic. All we care about is the remainder.
If x = 2, then 3*2 = 6 which leads to remainder 1 when we divide by 5. Therefore, 3x = 1 (mod 5) when x = 2.
If x = 3, then 3*3 = 9 = 4 (mod 5) because 9/5 = 1 remainder 4.
So 3x = 4 (mod 5) when x = 3.
The final answer is C) 3
We don't need to check D since x = 3 is a solution and it's smaller than x = 4.
If you wanted to check x = 4, then 3*4 = 12 = 2 (mod 5) because 12/5 yields a remainder of 2.