In the following question, four words are given, out of which one word is incorrectly spelt. Identify the incorrectly spelt word.

Introduction / Context:
This spelling question asks the candidate to identify the incorrectly spelt word from a list of four. Correct spelling is important both in exams and in real life communication, and commonly tested words often relate to everyday usage. The goal is to spot the extra or missing letters and match each word with its standard dictionary form.

Given Data / Assumptions:
- Words given: Impression, Personnal, Terrorism, Illiterate.
- Three of these are correctly spelt and one is incorrect.
- We must choose the incorrectly spelt word as the answer.
- The context is general English vocabulary.

Concept / Approach:
To answer spelling questions, we compare each word with its known correct form or break it into familiar parts. Impression, Terrorism and Illiterate are well known words that appear frequently in reading materials. The word Personnal appears suspicious because the usual spelling of the related adjective is Personal with a single n in the middle. The double n makes it incorrect as a common English word in this context, since Personnel is a different word meaning staff, and the given spelling does not match that either.

Step-by-Step Solution:
Step 1: Check Impression. The word impression is correctly spelt with im plus pression and is commonly used to mean an effect on mind or senses.Step 2: Examine Personnal. The standard adjective meaning relating to a person or private matters is personal, with one n after the r.Step 3: Note that Personnel, meaning staff or employees, is spelled with two n letters but also ends with nel, not nal. The given form Personnal does not match either correct word.Step 4: Terrorism is a correctly spelt word referring to the use of terror or violence for political ends.Step 5: Illiterate is correctly spelt and refers to someone who cannot read or write.Step 6: Therefore, Personnal is the incorrectly spelt word.

Verification / Alternative check:
We can verify using known correct spellings. Dictionaries list personal for private or individual, and personnel for staff. There is no standard word Personnal with that exact spelling in general English usage. The other words appear exactly as they are used in books and newspapers. This confirms that the error lies in the extra n in Personnal combined with the nal ending.

Why Other Options Are Wrong:
- Impression is a correctly spelt noun often used in phrases like good impression or first impression.
- Terrorism is spelled with double r and ism and is used widely in news reports and academic articles.
- Illiterate is correctly spelt with double l and means unable to read or write.

Common Pitfalls:
Spelling questions can be tricky when two similar correct words exist, such as personal and personnel. Candidates might confuse the patterns and feel uncertain. A good method is to remember that personal has one n and refers to private matters, while personnel has double n and refers to staff. When a mixed or hybrid spelling like Personnal appears, it should signal that something is wrong. Regular reading helps to store visual memory of correct word forms, which is more reliable than relying only on pronunciation.

Final Answer:
The incorrectly spelt word in the list is Personnal.