Explanation:

- Let's break down the code formation:

- 'sky planets satellites stars' -> 'od lk sk jk'

- 'sun moon space planets' -> 'mj jk dn ho'

- 'rocket stars sun airplane' -> 'gt fa mj lk'

- 'space earth sky rocket' -> 'sk mn ho gt'

- By cross-referencing:

- 'planets' is common in the first two sentences, matching code is 'jk'.

- 'stars' is common in the first and third sentences, matching code is 'lk'.

- 'sun' is seen in the second and third, matching code is 'mj'.

- 'space' is common in the second and fourth, matching code is 'ho'.

- 'sky' is present in the first and fourth, matching code is 'sk'.

- Hence, the matched code for ‘earth’ is not found in other sentences, hence it is 'mn'.

- The correct code for “earth” is:

- Option: 2 - Mn