When a snail is touched it retreats into its shell this is an example of?

The correct answer will be - sensitivity to external environment as response to stimulus.

Explanation:

The sensitivity to the external environment is a characteristic feature of a living organism.  

If we will touch a moving snail, the snail will retreat to its shell as a threat response. This is also known as the response to stimulus as touch acts as a stimulus which intimate cascade of reaction and causes retreat.  

If we continuously do this to snail then it will know that this is not a threat, therefore, it will continuously move. This process is known as habituation.

Thus, sensitivity to the external environment as a response to a stimulus is the correct answer.