Explanation:

Let's break down the code:

- ‘COW’ ? ‘ZRF’

- C ? Z: C (3rd letter) to Z (26th) ? move backwards by 3 (C?B?A?Z; cycling backwards: C-1=B, -2=A, -3=Z).

- O ? R: O (15th) to R (18th), move forward by 3.

- W ? F: W (23rd) to F (6th). Move forward by 9 (after Z back to A: W?X?Y?Z?A?B?C?D?E?F).

- ‘DOG’ ? ‘JRG’

- D ? J: D + 6.

- O ? R: O + 3.

- G ? G: Same letter.

Now, checking if first and third letters use a rule:

Try same letter positions for ‘COW’ and ‘DOG’:

- First: C?Z, D?J: Differs.

- Second: O?R (both), so always +3.

- Third: W?F (+9), G?G (+0).

So, only the middle letter follows a clear pattern: always +3.

Let’s try each option for ‘RAT’:

- R (18th), A (1st), T (20th).

- Middle: A+3 = D.

Options with D in middle:

- Option 2: WDU

- Option 3: UDW

Now, let’s check the full pattern:

Compare with the other patterns, but since only the middle one is clear (+3), the rest seems inconsistent.

So, by process of elimination and matching the pattern as best as possible, the most consistent is option 3: UDW

- R ? U (+3)

- A ? D (+3)

- T ? W (+3)

So the answer is:

- Option 3: UDW