In the given code, each letter is shifted by one position in the alphabet. For example:  
- G → H (shift by 1)  
- U → V (shift by 1)  
- N → O (shift by 1)  
Following the same pattern for PEN:  
- P → Q (shift by 1)  
- E → D (shift by 1)  
- N → P (shift by 1)  
Therefore, the code for PEN is "QFO." Hence, the correct answer is  "QFO."