The correct answer is - ক) Adverb of Reason.

• Reason or Purpose
- কোন কাজের কারণ বা উদ্দেশ্য বোঝাতে Adverb of Reason or Purpose হয়।
- এধরণের Adverb-গুলি হলো: Hence, therefore, so, so that ইত্যাদি।
Example:
- He therefore left the school.
- He is hence unable to deny it.
- He is ill so he cannot go to school.
- He works hard so that he can succeed.

• Other options:
Adverb of time:
- এই ধরনের Adverb যে গুলো ক্রিয়া সম্পন্ন হওয়ার সময় বুঝায়। এটি সাধারণত When কখন দিয়ে প্রশ্ন করলে উত্তর দেয়।
- Example: Now, then, before, since, ago, already, once, twice, thrice, again, soon, late, afterwards, yesterday, today, tomorrow, daily. early, formerly, frequently. presently, immediately, instantly.
Example Sentence: They arrived yesterday at the station.

Adverb of manner:
- যে adverb কোনো কিছু কিভাবে হয় হচ্ছে তা উল্লেখ করে তাকে Adverb of manner বলে।
- Example: sadly, softly, steadily, slowly, soundly, swiftly, simply, suddenly, carefully, carelessly, easily, quickly, possibly, probably, luckily, fortunately, unfortunately, naturally, rightly, urgently, wrongly.
Example Sentence: He spoke quickly during the meeting.

Adverb of degree:
- এই ধরনের adverb কতোখানি, কি মাত্রায় এসব বোঝায়।
- It tells us about the intensity of something.
Example: Almost, quite, very, much, fully, partly, wholly, completely, strongly, totally, entirely, deeply, greatly, poorly, half, enough, somewhat, altogether, too, little, a little, rather.
Example Sentence: The water is completely cold.

Source: Applied English Grammar and Composition by P. C. DAS.