The correct option is C cannot be analysed
Option A: It's mentioned in this line: "a spelling system that defies analysis."
By the phrase 'defies analysis', we mean that 'it is difficult or challenging to analyze'. ('defy' means 'challenging')
It simply means that the spelling system cannot be analyzed.
Hence option A is correct.
Option B: 'Cannot be understood' is not a suitable option, as these words have not been written in relation to the 'spelling system'.
According to these lines, 'system of weights and measures that is intelligible only to the compiler of arithmetic books', 'intelligible' means 'comprehensible, understandable', and relates to the system of weights and measures.
Hence option B is incorrect.
Option C: This line does not convey what the line in the passage means.
'Cannot be found' refers to something that is lost, and the spelling system isn't lost. So, option C is incorrect.
Option D is incorrect because the answer lies in A.