Explanation:

- The phrases and their codes provide the basis to decode words.

- The phrase ‘they were not playing’ translates to ‘su to al zi’.

- ‘They are singing tomorrow’ corresponds to ‘to ej di mo’.

- The shared code ‘to’ in both indicates ‘they’.

- ‘Are you playing’ is ‘su mo pa’, sharing the code ‘su’ with the first phrase, suggesting 'playing’.

- ‘You were sleeping’ translates to ‘pa zi qu’.

- Comparing it with ‘they were not playing’, the common code ‘zi’ points to ‘were’.

- Therefore, ‘qu’ is uniquely associated with ‘sleeping’.

Based on the analysis, the code 'qu' stands for sleeping.

Correct Answer: Option 2: sleeping

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