Select the INCORRECTLY spelt word.

The correct answer is Cheerfull.

Let us analyze the spelling of the given words:
1. Cheerfull: This word is INCORRECTLY spelt. The correct spelling is Cheerful (with a single 'l' at the end). It means full of cheer, happy, or optimistic.
2. Tsunami: Correctly spelt. It refers to a long high sea wave caused by an earthquake or other disturbance.
3. Efficient: Correctly spelt. It means achieving maximum productivity with minimum wasted effort or expense.
4. Responsibilities: Correctly spelt. It is the plural form of responsibility, meaning state or fact of having a duty to deal with something.

Therefore, the incorrectly spelt word among the given options is Cheerfull.