Looking at the code transformation:
- 'F' is replaced by 'O'
- 'R' is replaced by 'F'
- 'O' is replaced by 'A'
- 'Z' is replaced by 'Q'
- 'E' is replaced by 'T'
- 'N' is replaced by 'H'
The pattern is: the first letter of 'FROZEN' is mapped to 'O', the second letter to 'F', and so on.
Now, applying the same pattern to 'MOLTEN':
- 'M' corresponds to 'O'
- 'O' corresponds to 'F'
- 'L' corresponds to 'U'
- 'T' corresponds to 'M'
- 'E' corresponds to 'P'
- 'N' corresponds to 'N'
Therefore, the correct code for 'MOLTEN' is "OFUMPN".
Thus, the correct answer is \( \boxed{3} \).