In the following spelling based English question, four words are given and you must identify which word is incorrectly spelt among the options.

Introduction / Context:
Spelling based questions require you to distinguish between correctly and incorrectly spelt English words. In this item, you are given four words and asked to select the one that is spelt wrongly. Such questions are common in competitive exams and help test your familiarity with standard English spelling patterns, especially in words derived from Greek or Latin.

Given Data / Assumptions:

• Four words are provided: Declension, Labyrinth, Quintessence, and Pandemonioun.
• Three of these are correctly spelt common English words.
• One word contains incorrect letters or letter order.
• Your task is to identify the incorrectly spelt word.

Concept / Approach:
To solve such questions, you should first check which words you recognise from reading and usage. "Declension" is correctly spelt and refers to the variation of the form of a noun, pronoun, or adjective. "Labyrinth" is the correct spelling for a maze like structure. "Quintessence" is correctly spelt and means the purest or most perfect example of something. The troublesome word is "Pandemonioun", which appears to be a misspelling of "Pandemonium", the correct English word for wild uproar or chaos. Recognising the standard form of this word leads you to identify "Pandemonioun" as incorrect.

Step-by-Step Solution:
Step 1: Examine each option and check whether you have seen the word before in books, articles, or exams. Step 2: Confirm that "Declension" is a known grammatical term and its spelling matches the standard form. Step 3: Confirm that "Labyrinth" is the correct spelling for a maze like structure, with "rynth" at the end. Step 4: Verify that "Quintessence" is correctly spelt, with the sequence "qui-nt-ess-ence". Step 5: Notice that "Pandemonioun" looks suspicious and compare it with the familiar word "Pandemonium", realising that the ending has been incorrectly written as "ioun" instead of "ium".

Verification / Alternative check:
To further verify, you can recall typical uses of the word "pandemonium" in sentences, such as "When the announcement was made, there was pandemonium in the hall." In print, this word always appears with the ending "-monium". The presence of "-nioun" in "Pandemonioun" clearly deviates from this standard spelling. On the other hand, checking dictionaries or your reading memory for "Declension", "Labyrinth", and "Quintessence" confirms that these match their correct forms.

Why Other Options Are Wrong:

• Option A, "Declension", is correctly spelt and is a recognised grammatical term.
• Option B, "Labyrinth", is correctly spelt and widely used to describe complex maze like structures.
• Option C, "Quintessence", is also correctly spelt and is a common advanced vocabulary word meaning the purest essence.

Common Pitfalls:
Spelling questions are tricky because all the options can look strange under exam pressure. Students sometimes doubt correct words like "Labyrinth" because the "y-r-i-n" sequence seems unusual. Another error is to be guided purely by appearance rather than by memory of seeing the word in context. To improve performance, read widely and pay attention to the spelling of advanced words, especially those ending in "-ium", "-ence", or "-tion". This repeated exposure builds a mental pattern that helps you quickly spot incorrect forms like "Pandemonioun".

Final Answer:
The incorrectly spelt word among the four options is Pandemonioun, which should correctly be spelt as "Pandemonium". Therefore Option D is correct.