Select the INCORRECTLY spelt word.

To find the incorrectly spelt word, let's analyze the spelling and meaning of each option:

1. Passage: Correctly spelt. It refers to a path, channel, or a portion of a written work.
2. Reconsidered: Correctly spelt. It is the past tense of "reconsider," meaning to think about something again, especially for a possible change of decision.
3. Prevention: Correctly spelt. It refers to the act of stopping something from happening or arising.
4. Supremecy: Incorrectly spelt. The correct spelling is Supremacy (ending with "-acy", not "-ecy"). It refers to the state or condition of being superior to all others in authority, power, or status.

Therefore, the incorrectly spelt word is "Supremecy".