In this English spelling question, four words are given and you must choose the one that is incorrectly spelt when compared with standard English spelling conventions.

Introduction / Context:
This question tests your ability to recognise correct and incorrect English spellings. In competitive exams, spelling questions are often used to check careful reading and familiarity with commonly used words. Here, four words are presented, and you must identify the one which is not spelt according to standard English usage.

Given Data / Assumptions:
- Four options are given: "Gaurantee", "Itinerary", "Magnificent", and "Writing".
- Exactly one of these words has an incorrect spelling.
- The words are of moderate frequency in written English, especially in formal and travel related contexts.

Concept / Approach:
To solve spelling questions, you should recall the correct spelling of familiar words or break them into syllables. The word "Guarantee" is commonly misspelt, so it is important to know its correct letter sequence. "Itinerary", "Magnificent", and "Writing" are standard English words whose spellings should already be familiar from textbooks and formal usage.

Step-by-Step Solution:
Step 1: Analyse "Gaurantee". The correct spelling of this word is "Guarantee", with the sequence g-u-a-r-a-n-t-e-e. Step 2: Observe that in "Gaurantee", the letters after "g" are arranged as g-a-u-r rather than g-u-a-r, which makes the word incorrect. Step 3: Check "Itinerary". This is correctly spelt as i-t-i-n-e-r-a-r-y, commonly used for a travel schedule. Step 4: Check "Magnificent". This is correctly spelt and is a common adjective meaning grand or impressive. Step 5: Check "Writing". This is also correctly spelt and is the standard form of the verb and noun related to writing.

Verification / Alternative check:
If you consult any standard dictionary, you will find the entry "Guarantee" with the letter order g-u-a-r-a-n-t-e-e. The incorrect option "Gaurantee" swaps the positions of "u" and "a" near the beginning, which is a well-known spelling error. The other three options exactly match their standard dictionary spellings. Therefore, only "Gaurantee" can be accepted as the incorrectly spelt word in this set.

Why Other Options Are Wrong:
"Itinerary" is correct because it follows the standard spelling used in travel documents and timetables. "Magnificent" is correct and is widely used to describe something very beautiful or impressive. "Writing" is also correct and is one of the most common English words. None of these contain letter order errors or extra letters, so they cannot be chosen as incorrect spellings.

Common Pitfalls:
Candidates sometimes hesitate between similar looking spellings, especially for longer or less frequently used words like "Itinerary" and "Guarantee". A useful technique is to break words into familiar parts or recall how they look in official documents. For instance, the "guar" part in "Guarantee" stays together and should not be broken into "gaur". Regular reading and writing practice greatly reduce such spelling mistakes over time.

Final Answer:
The incorrectly spelt word in this set is Gaurantee, which should correctly be written as "Guarantee".