Correct Answer: ru

Solution : Given:
1. honest persons ⇒ yu du
2. he is honest ⇒ du mu cu
3. she is emotional ⇒ mu hu ru
4. she left ⇒ hu nu

By comparing all the coded sentences, we find that –
In sentences 1 and 2, honest is a common word and du is a common code.
In sentences 2 and 3, is is a common word and mu is a common code.
In sentences 3 and 4, she is a common word and hu is a common code.
The remaining word and code in sentence 1 persons and yu ; in sentence 2 he and cu ; in sentence 3 are emotional and ru ; in sentence 4 left and nu .
Finally, honest ⇒ du; is ⇒ mu; she ⇒ hu; ⇒ persons ⇒ yu; he ⇒ cu; emotional ⇒ ru; left ⇒ nu

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