Public key encryption relies on a one-way function, which is easy to do in one direction but hard to do in reverse. Which of the following functions would best represent a one-way function? In other words, given a known input and a known output, which function would it be hardest to guess the secret number?

1. Correct Answer
   Incorrect Answer
   a

   input * secret number = output

2. Correct Answer
   Incorrect Answer
   b

   input MOD secret number = output

3. Correct Answer
   Incorrect Answer
   c

   input + secret number = output

4. Correct Answer
   Incorrect Answer
   d

   input raised to the power of secret number = output