This question involves a simple coding or decoding pattern where digits represent words.

Decoding the Code

We are given three coded sentences:

• '178' means 'read history books'
• '586' means 'they made history'
• '741' means 'read good books'

Our goal is to find the digit that represents the word 'made'.

Step 1: Identify Common Words and Digits Between Sentences

• Compare '178' ('read history books') and '586' ('they made history'). The common word is 'history', and the common digit is '8'. Therefore, '8' stands for 'history'.
• Compare '178' ('read history books') and '741' ('read good books'). The common words are 'read' and 'books'. The common digits are '1' and '7'. This means '1' and '7' together represent 'read books'.

Step 2: Deduce the Code for 'made'

• Now consider the second sentence: '586' means 'they made history'.
• We know '8' means 'history'.
• Removing '8' and 'history', we are left with '56' meaning 'they made'.
• The words 'they' and 'made' correspond to the digits '5' and '6'.
• Since we don't have more information to distinguish between 'they' and 'made' for '5' and '6', the word 'made' could be represented by either '5' or '6'.

Conclusion

Based on the deductions, the digit for 'made' could be either '5' or '6'. This corresponds to option D.