Explanation:

- Let's decode the phrases using the given code words:

- 'they were not playing' = 'su to al zi'

- 'they are singing tomorrow' = 'to ej di mo'

- 'are you playing' = 'su mo pa'

- 'you were sleeping' = 'pa zi qu'

- From the above, we can deduce the following:

- 'they' = 'to', 'were' = 'zi', 'playing' = 'su'

- 'are' = 'mo', 'you' = 'pa', 'singing' = 'ej'

- 'sleeping' = 'qu', 'tomorrow' = 'di'

- The phrase 'singing and playing' is coded as ‘ej su gs’.

- We know 'singing' = 'ej' and 'playing' = 'su'.

- Therefore, 'and' = 'gs'.

- The code for 'and' is Option 4: gs.