He was so weak that he could not walk.
Here, the underlined clause is -

• Adverbial Clause of Result:
Principal Clause-এর result বা ফল প্রকাশ করলে সেটি Adverbial Clause of Result হয়।
এ ধরনের clause সাধারণত so ... that বা such ... that structure-এ ব্যবহৃত হয়।
• প্রদত্ত sentence-এ “that he could not walk” clause টি weak হওয়ার ফল প্রকাশ করছে।
- তাই এটি Adverbial Clause of Result.

Correct Answer: He was so weak that he could not walk.
Here, the underlined clause is - Adverbial Clause.

Source:
1. A Passage to the English Language, S.M. Zakir Hussain.
2. English Wizard by Live Publications.