In a certain code, '247' means 'spread red carpet' ; '256' means 'dust one carpet' and '234' means 'one red carpet'. Which digit in that code means 'dust' ?

In the first and second statements, the common code digit is '2' and the common word is 'carpet'.

So, '2' means 'carpet'.

In the second and third statements, the common code digit is '6' and the common word is 'one'.

So, '6' means 'one'.
Therefore, in the second statement, '5' means 'dust'.