He was so tired that he fell asleep immediately.
Here, the underlined clause is -

• Adverbial clause of result:
- Principal clause এর result বা ফল প্রকাশ করা।
- Adverbial Clause of reason টি That Conjunction দিয়ে শুরু হয়।
- এ Clause সাধারণত Main clause এ ব্যবহার করা so/ much adverb কে modify করে।

Correct Answer: He was so tired that he fell asleep immediately. Here, the underlined clause is - Adverbial Clause.

Source: Advanced Learner's Communicative English Grammar & Composition By Chowdhury & Hossain.