Select the INCORRECTLY spelt word.

The incorrectly spelt word is Occured.

Step-by-step analysis of the options:

1. Equally: This word is spelt correctly. It is an adverb meaning in equal amounts or to an equal degree.
2. Unwilling: This word is spelt correctly. It is an adjective meaning not ready, eager, or prepared to do something.
3. Argument: This word is spelt correctly. It is a noun referring to an exchange of diverging or opposite views.
4. Occured: This word is incorrectly spelt. The correct spelling is Occurred, which requires a double 'r'.

Hence, the incorrectly spelt word is Occured.