Question

No offence intended _______ I think you haven't understood the problem correctly. 
(a) when 
(b) if 
(c) as 
(d) but

Answer 1

The answer will be option d) but. "No offence intended, but I think you haven't understood the problem correctly.

but is a coordinating conjunction. The rest are subordinating conjunctions and relative pronouns.

They are not applicable to the above sentence. The appropriate subordinating conjunction would be "however" in this sentence.

Answer 2

✳ option d is correct, but is used here in the blank.