Answer: the thousands period.
Step-by-step explanation:
1) The complete number is: 913,256.
2) Each group of three digits forms a period.
3) The first group of three digits, 6, 5, and 2 are 6 ones, 5 tens, and 2 hundreds. This group of numbers belong to the ones period.
4) The second group of three digits is 3, 1, and 9, which means: 3 thousands, 1 ten thousands, and 9 hundred thousands. This group is called the thousands period.