25/100 is 1/4 in simplest form. When you find simplest form of a fraction, divide both the numerator and denominator by something that you can divide both of them evenly by and get whole numbers as a result. In this case, both of them are divisible by 25. After you divide, you get 1/4. Is 1/4 able to be simplified further? No, so 1/4 is the simplest form.
Hope that helped!