Select the INCORRECTLY spelt word.

The correct option is Atrocios.

Let's analyze the spelling of each given option:
1. Accessibility: Spelt correctly. It refers to the quality of being easy to approach, reach, enter, or use.
2. Apparatus: Spelt correctly. It refers to the technical equipment or machinery needed for a particular activity or purpose.
3. Aptitude: Spelt correctly. It refers to a natural ability, suitability, or talent.
4. Atrocios: Spelt incorrectly. The correct spelling is Atrocious (with a 'u' before the 's'). It means extremely wicked, brutal, or of very poor quality.