The correct answer is:
The thousands period.
Explanation:
We read the number 913,256 (given in the comments) as "nine hundred thirteen-thousand, two hundred fifty-six." The 913 grouping is in the part ending with "thousand", so it is the thousands period.
Looking at it from place value, 9 is in the hundred-thousands place; 1 is in the ten-thousands place; and 3 is in the thousands place. This makes this group in the thousands period.