Correct Answer: sum

Solution : Given:
1. sky is blue ⇒ mud pot sum
2. I like blue ⇒ mud man log
3. I am sky ⇒ man red pot

By comparing all the coded sentences, we find that –
In sentences 1 and 2, blue is a common word and mud is a common code.
In sentences 1 and 3, sky is a common word and pot is a common code.
In sentences 2 and 3, I is a common word and man is a common code.
The remaining words and codes in sentence 1 are is and sum ; in sentence 2 are like and log ; in sentence 3 are am and red .
Finally, blue ⇒ mud; sky ⇒ pot; I ⇒ man; is ⇒ sum; like ⇒ log; am ⇒ red.

So, is will be coded as sum . Hence, the first option is correct.