Select the INCORRECTLY spelt word.

The correct answer is ettiquete.

Explanation:
Let's analyze the given options to identify the incorrectly spelt word:

1. ettiquete: This word is incorrectly spelt. The correct spelling is etiquette (which means the customary code of polite behavior in society or among members of a particular profession or group).

2. rationalisation: Correctly spelt. It refers to the action of attempting to explain or justify behavior or attitudes with logical reasons.

3. elopement: Correctly spelt. It refers to the act of running away secretly to get married.

4. occasion: Correctly spelt. It refers to a particular event, or a time at which an event takes place.