Step-by-step Solution:

Correct Answer: C (tr)

To find the code for 'blue', we can compare the given sentences and their codes to find common words and their corresponding codes.

Step 1: List the Given Information

Let's write down the sentences and their codes clearly:

1. 'blue is pale' → 'tr mn jo'
2. 'red but blue' → 'ce bd tr'
3. 'apple is red' → 'bd gf mn'

Step 2: Compare Sentences 1 and 2

We look for common words and codes between the first two sentences.

• Sentence 1: 'blue is pale'
• Sentence 2: 'red but blue'
• The only common word is 'blue'.

Now, let's look at their codes:

• Code 1: 'tr mn jo'
• Code 2: 'ce bd tr'
• The only common code is 'tr'.

Since 'blue' is the only common word and 'tr' is the only common code, we can conclude that the code for 'blue' is 'tr'.

Step 3: Confirm with Other Sentences (Optional)

We can continue this process to be certain and find the codes for other words.

• Comparing Sentences 2 and 3: The common word is 'red' and the common code is 'bd'. So, 'red' = 'bd'.
• Comparing Sentences 1 and 3: The common word is 'is' and the common code is 'mn'. So, 'is' = 'mn'.

Conclusion

By comparing the first two sentences, we definitively find that the code for 'blue' is 'tr'.