When finding the simplest form of a fraction, think of the GCF, or greatest common factor between the two numbers (100 and 1000)
In this case, it would be 100 as the greatest number they are divisible by is 100.
Then, to reduce the fraction, divide the top and bottom number by the GCF.
100/1000=1/10
The simplest form is 1/10
Hope this helped :)