In a certain code language, 'careful emotion' is coded as 'du yu', 'sun key' is coded as 'bin fin', 'careful sun' is coded as 'du bin' and 'emotion key' is coded as 'yu fin'. What is the code word for 'sun'?

To find the code for 'sun', we can compare the given statements:
'careful emotion' = 'du yu'

'sun key' = 'bin fin'

'careful sun' = 'du bin'

'emotion key' = 'yu fin'

Comparing statements (2) and (3):
'sun key' = 'bin fin'
'careful sun' = 'du bin'

The common word in both statements is 'sun', and the common code is 'bin'.
Therefore, the code for 'sun' is 'bin'.