Select the INCORRECTLY spelt word.

The correct answer is Advertisment.

Explanation:

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

1. Advertisment: This is incorrectly spelt. The correct spelling is Advertisement (it requires an 'e' between 's' and 'm'). It means a notice or announcement in a public medium promoting a product, service, or event.

2. Adulterous: This spelling is correct. It relates to or involves adultery.

3. Astounding: This spelling is correct. It means surprisingly impressive or notable.

4. Amendment: This spelling is correct. It means a minor change or addition designed to improve a text, piece of legislation, etc.

Therefore, the word with the incorrect spelling is Advertisment.