The pH of a solution changes from 5.5 to 3.5.

Question

This indicates that the [OH−] of the solution
has
1. increased by a factor of 100.
2. increased by a factor of 2.
3. decreased by a factor of 2.
4. decreased by 2 M.
5. decreased by a factor of 100.
6. increased by 2 M.

Answer 1

5 is the answer.
5.5 to 3.5 the solution has become more acid by pH of 2 and since this is a log term (pH = -log(H^+) that's more acidic by a factor of 100.
So OH must have decreased by a factor of 100.