The numbers are relatively prime if there is no common factor that divides both of them apart from 1. A) isn't answer because 3 is one of common factors. B isn't answer because 13 is common factor. and D isn't answer because 7 is common factor.
Therefore only 10 and 31 have no common factors other than 1
Keep doing each step manually, and see where it slips up:
−7.9+8.2−3.4+2.1=−1
−7.9+8.2+(−3.4)+2.1=−1
−7.9+(−3.4)+8.2+2.1=−1
(−7.9+(−3.4))+(8.2+2.1)=−1
−11.3+10.3=−1
