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 requires you to find the incorrectly spelt word among four options. Spellings of longer words with Latin or French origins are often tested because they can be confusing. Accurate spelling is essential for formal writing, and developing an eye for common patterns helps greatly in examinations.

Given Data / Assumptions:
- Words given: Stupefaction, Preferential, Surveillence, Detrimental.
- Exactly one of these words is incorrectly spelt.
- We must select the incorrect spelling as the answer.
- The words come from typical academic or formal vocabulary.

Concept / Approach:
The correct spelling of the word meaning close observation, especially of a person or place, is surveillance. The given option Surveillence replaces the a with an e near the end, which makes it incorrect. The other words match their standard dictionary forms. Stupefaction, Preferential and Detrimental are commonly seen in academic texts and news articles with the spellings shown in the options.

Step-by-Step Solution:
Step 1: Check Stupefaction. This is correctly spelt and refers to a state of being stupefied or astonished.Step 2: Examine Preferential. It is correctly spelt and describes treatment that gives preference to someone or something.Step 3: Look at Surveillence. The standard English word is surveillance, with the ending ance, not ence.Step 4: Detrimental is correctly spelt and means harmful or damaging.Step 5: Since Surveillence differs from the correct form surveillance, it is the incorrectly spelt word.

Verification / Alternative check:
A quick check with known similar words helps. Many English nouns that refer to an action or process end with ance, such as performance, guidance and acceptance. Surveillance follows this pattern. The variant with ence is not used for this particular word. The other words, Stupefaction, Preferential and Detrimental, all agree with their known dictionary spellings, confirming that they are correct and that the error is confined to Surveillence.

Why Other Options Are Wrong:
- Stupefaction is correctly spelt and is derived from stupefy.
- Preferential is correct and commonly seen in phrases like preferential treatment.
- Detrimental is correct and used widely to describe harmful effects.

Common Pitfalls:
Words borrowed from French, such as surveillance, often retain spellings that can confuse learners, especially the pattern of vowels and double consonants. It is easy to misplace a or e when writing quickly. To reduce errors, it helps to note common suffix patterns like ance versus ence for specific words and to practice writing them. Visual memory from regular reading is also a strong support, since the correct forms appear repeatedly in quality texts.

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