Select the INCORRECTLY spelt word.

The correct answer is Symtoms.

Let us examine the spelling and meaning of each option to identify the misspelled word:
1. Progress - Correctly spelt. It means forward or onward movement toward a destination or goal.
2. Mystery - Correctly spelt. It refers to something that is difficult, strange, or impossible to understand or explain.
3. Pilgrim - Correctly spelt. It refers to a traveler who journeys to a sacred place for religious reasons.
4. Symtoms - Incorrectly spelt. The correct spelling is Symptoms (with a 'p'), which refers to physical or mental features that indicate a condition of disease.

Therefore, the incorrectly spelt word among the given choices is "Symtoms".