'smart work earn reward' is written as ' ab ir ha ig' ------------- (1)
'earn get reward to fame ' is written as 'af ig am ir sr' ------------- (2)
'salary to get work now' is written as 'am oj on ab af ' ------------- (3)
'to better life earn' is written as 'ik ir ot af' ------------- (4)
From 1,2,4 : 'earn' is 'ir'
From 1,2 : 'reward' is 'ig'
From 1,3 : 'work' is 'ab'
From 2,3 : 'get' is 'am'
From 2,3,4 : 'to' is 'af'
From 1 : By elimination, 'smart' is 'ha'
From 2 : By elimination, 'fame' is 'sr'
From 3 : By elimination, 'salary' is 'oj/on'
From 3 : By elimination, 'now' is 'oj/on'
From 4 : By elimination, 'better' is 'ik/ot'
From 4 : By elimination, 'life' is 'ik/ot'
Summary :
| earn | Ir | 
| reward | Ig | 
| work | Ab | 
| smart | Ha | 
| to | Af | 
| get | Am | 
| fame | Sr | 
| salary/now | oj/on | 
| better/life | ik/ot | 
In the given code language, the code for 'better/life' is 'ik/ot'. And 'quickly' is not given in the language. So according to the question, the possible code is 'ij' because 'ij' is also not given in the language as a code for any other words.