In a certain code language, '361' means 'food and water'; '678' means 'buy good food' and '723' means 'buy clean water'. Which of the following digits stands for 'clean'?

To find out which digit stands for 'clean', we need to compare the codes for 'buy good food' and 'buy clean water'.

The code for 'buy good food' is '678' and the code for 'buy clean water' is '723'.

The common word in both sentences is 'buy' and the common digit in both codes is '7'. So, '7' stands for 'buy'.

Now, let's compare 'buy good food' and 'food and water'. The common word is 'food' and the common digit is '6'. So, '6' stands for 'food'.

By process of elimination, in the code '723' for 'buy clean water', '7' stands for 'buy', '6' stands for 'food', so '2' must stand for 'clean'.

So, the digit that stands for 'clean' is '2'.