Question : In a certain code, '13' means 'stop smoking' and '59' means…

In a coding-decoding data-sufficiency question, when a newly coded phrase shares a digit with an already-known coded phrase, the plain-text meaning shared between the two phrases must correspond to that shared digit, since the digit-to-word correspondence is one-to-one and consistent within the code. Testing one new clue against the base code this way can, on its own, pin down an unknown digit's meaning.

Checking Statement I ('157' means 'stop bad habit') against the base codes:

1. '59' means 'injurious habit' and '157' means 'stop bad habit'; the digit common to both codes is '5', and the word common to both meanings is 'habit'.

2. So '5' stands for 'habit'.

3. The remaining digit in '59', which is '9', must then stand for the remaining word, 'injurious'.

4. Statement I alone is therefore enough to fix both '5' = 'habit' and '9' = 'injurious'.

Checking Statement II ('839' means 'smoking is injurious') against the base codes:

1. '13' means 'stop smoking' and '839' means 'smoking is injurious'; the digit common to both codes is '3', and the word common to both meanings is 'smoking'.

2. '59' means 'injurious habit' and '839' means 'smoking is injurious'; the digit common to both codes is '9', and the word common to both meanings is 'injurious'.

3. So '9' stands for 'injurious', and the remaining digit in '59', which is '5', stands for the remaining word, 'habit'.

4. Statement II alone is therefore also enough to fix both '9' = 'injurious' and '5' = 'habit'.

Both routes agree on the same decoding -- '5' means 'habit' and '9' means 'injurious' -- so the two statements corroborate each other independently rather than needing to be combined.

Since Statement I alone fixes both meanings, and Statement II alone also fixes both meanings via a different pair of comparisons, either statement independently is sufficient to answer the question.