6/10 is the closest to zero because if you simplify all the fractions you can, then convert them into decimals, you can easily find what is closer to zero:
8/12 = 2/3 = 0.66
5/8 = 0.62
6/10 = 3/5 = 0.60
2/3 = 0.66
0.60 or 6/10 is the smaller number ;))