Correct answer for: No offence intended ... I think you haven't understood the...

[Sultano]

Could anybody give me the correct answer giving me the reason?
" No offence intended ........ I think you haven't understood the problem correctly."
a) if b) when c) but d) as

[Mister Micawber]

.
'But', because the 2 clauses are in contrast.
.
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Native English teacher at Mister Micawber's