Correct Answer: pi

Solution : Given:
1. rocket aeroplane sky stars ⇒ ci pi di ki
2. satellite moon sun sky ⇒ ni ci ui li
3. sky sun stars mars ⇒ ui gi ki ci
4. venus aeroplane moon planet ⇒ di qi mi ni

By comparing all the three coded sentences, we find that –
In sentences 1 and 4, the common word is aeroplane ; the common code is di
In sentences 1, 2, and 3, the common word is sky ; the common code is ci
In sentences 1 and 3, the common word is stars ; the common code is ki
In sentences 2 and 4, the common word is moon ; the common code is ni
In sentences 2 and 3, the common word is sun ; the common code is ui
The remaining words and code in sentence 1 are rocket and pi; in sentence 2 are satellite and li ; in sentence 3 are mars and gi ; in sentence 4 are venus / planet and qi / mi
Finally, aeroplane ⇒ di; sky ⇒ ci; stars ⇒ ki; moon ⇒ ni; sun ⇒ ui; rocket ⇒ pi; satellite ⇒ li; mars ⇒ gi; venus/ planet ⇒ qi/ mi

Therefore, the code for rocket is pi . Hence, the third option is correct.