Step 1 (Understand the sentence structure).
The first part "I will attend the meeting" is contrasted with the second part "I have to finish this report first". This indicates a contrast rather than cause or result.
Step 2 (Check each option).
(a) "so" — would imply result, not contrast.
(b) "but" — shows contrast between the intention to attend the meeting and the immediate need to finish the report.
(c) "because" — indicates reason, which doesn't fit naturally here.
(d) "nor" — used for negative addition, not applicable here.
Step 3 (Construct the sentence).
"I will attend the meeting, but I have to finish this report first." — correct meaning and flow.
\[
{\text{but (Option (b)}}
\]