In this spelling question, four words are given and you must identify the one that is incorrectly spelt according to standard English usage.

Introduction / Context:
This question tests your ability to recognise correct English spellings, a skill that is essential for error-spotting, descriptive writing and even successfully filling application forms. The four options contain everyday words, and only one has a spelling mistake. Your task is to pick the incorrectly spelt word so that you can mentally correct it and remember the proper form for future use.

Given Data / Assumptions:
- Options: 'Forfeit', 'Gorilla', 'Blissfull', 'Corrupt'.
- Exactly one option is incorrectly spelt.
- Standard British or international English spellings are assumed for exam purposes.

Concept / Approach:
To solve spelling questions, you should recall known words from reading or break them into root plus suffix. The correct spelling of the adjective meaning 'full of bliss' or 'very happy' is 'Blissful' with a single 'l' at the end. Many English adjectives end with '-ful', not '-full' (for example, 'careful', 'useful', 'hopeful'). The other options—'Forfeit', 'Gorilla' and 'Corrupt'—are common words that are correctly spelt.

Step-by-Step Solution:
Step 1: Examine 'Forfeit'. This is correctly spelt as f-o-r-f-e-i-t and means a penalty or loss due to wrongdoing. Step 2: Examine 'Gorilla'. This is correctly spelt for the large ape and is commonly seen in textbooks and documentaries. Step 3: Examine 'Blissfull'. This appears to end with '-full', which is suspicious because many adjectives take the suffix '-ful'. Step 4: Examine 'Corrupt'. This is correctly spelt for dishonest or immoral behaviour, especially in politics or business. Step 5: Confirm that the correct spelling of 'Blissful' contains one 'l' at the end, not two.

Verification / Alternative check:
Check the pattern for other similar adjectives: 'joyful', 'beautiful', 'cheerful', all end in '-ful' with one 'l'. 'Blissfull' breaks this regular pattern and should instead be written as 'Blissful'. The spellings of 'Forfeit', 'Gorilla' and 'Corrupt' match standard dictionary forms. Therefore, 'Blissfull' is the only word with a spelling error among the options.

Why Other Options Are Wrong:
'Forfeit' is not the incorrect spelling because it correctly uses 'feit' at the end. 'Gorilla' is correct and should not be confused with 'guerrilla', which is a different word meaning a type of fighter. 'Corrupt' is also correctly spelt and commonly used with this exact letter sequence. Since only 'Blissfull' has an extra 'l', it stands out as the misspelt word.

Common Pitfalls:
One frequent error is to assume that because a noun like 'full' has two 'l' letters, the adjective suffix must also be '-full'. But in standard English, the correct suffix is '-ful', with just one 'l'. Another pitfall is second-guessing familiar words like 'Gorilla' or 'Corrupt' because you have seen them less often in written form. Regular reading of quality material such as newspapers and books helps you internalise correct spellings and reduces this uncertainty.

Final Answer:
The incorrectly spelt word is Blissfull; it should be correctly written as 'Blissful'.