Answer: ones.
Step-by-step explanation:
1) The group of the first three digits (those most to the right) of a number belong the period of the ones.
2) The second grup of three digits (tho the left of the first group) belong to the of the thousands.
3) The next group of three digist to the left belong to the period of the millions.
4) The next three form the group of the billions.
5) The other three are the trillions.
6) Therefore, for the given number 751,486, the digits 486 are in the period of the ones, and the digits 721 are in the period of the thousands.