The name of period that has 751 in the number
is hundreds.
Because Period is considered to a group of places of the
digits.
In this case, the given number is 751 and it is the group of hundred because 700 is the highest number it contains.
We can also write 751 as
751 = 700 + 50 + 1
It means,
7 hundreds + 5 tens + 1 ones