Pick out the incorrectly spelt word:

To identify the incorrectly spelt word, evaluate each option for standard English spelling.

Option A: Enunciate – Spelled E-N-U-N-C-I-A-T-E. This is the correct spelling of the word meaning to speak clearly.

Option B: Gullible – Spelled G-U-L-L-I-B-L-E. This is the correct spelling of the word meaning easily deceived.

Option C: Infallible – Spelled I-N-F-A-L-L-I-B-L-E. This is the correct spelling of the word meaning incapable of making mistakes.

Option D: Instegate – This is not a valid English word. The intended word may be 'Instigate' or 'Institute', but 'Instegate' is misspelled.

Thus, the incorrectly spelt word is Option D.