Explanation:

To understand why the correct answer is 'little', we need to examine the meaning and usage of the word 'little' in this context.

Meaning of 'little':
In this context, 'little' is used as a determiner to describe a small quantity or amount of something. It indicates that there is not much of the item in question.

Usage of 'little':
When we say "There is only little milk left in the bottle," we are implying that there is not much milk remaining. It suggests that the quantity of milk left is small or insufficient for a particular purpose.

Comparison with other options:
- 'Few': 'Few' is used to describe a small number of countable items. However, milk is not countable, so we cannot use 'few' in this context.
- 'A little': 'A little' is used to describe a small amount of something, but it implies that the amount is enough or sufficient. In this case, the sentence suggests that the amount of milk left is not sufficient, so 'a little' is not appropriate.
- 'Many': 'Many' is used to describe a large number or a large amount. Since the sentence indicates that there is only a small quantity of milk left, 'many' is not the correct option.

Conclusion:
The correct answer is 'little' because it accurately describes the small quantity or amount of milk remaining in the bottle. It follows the rules of usage and meaning for this specific context.