His fondness _ makes his writings difficult to understand because most readers don't know the knewly invented words.

Explanation

The correct answer is neologism, which means a newly coined word or expression. This matches the phrase 'newly invented words' used in the question. Archaic words are old and rare in modern usage, hence not newly invented. Sking appears to be a spelling error and does not relate to the question's context. Advanced words are complex or sophisticated but not newly invented, making them an incorrect choice here.