C.) 15,600
Since you are using letters as your password and you can't use each letter more than once, you have to do 26×25×24.
26 first because there are 26 letters in the alphabet, then 25 because you already used one letter and can't use it again, then 24 because you used two letters that can't be reused. Multiply those together and you should get 15,600.
Hope this helps!