He sank back on his pillow and fell into a __________.

• শূন্যস্থানে সঠিক উত্তর - brown study.
- Complete sentence: He sank back on his pillow and fell into a brown study.

• Brown study
English Meaning: absorbed in one's thoughts and not paying attention to anything else.
Bangla Meaning: তীব্র নিমগ্নতা বা বিমূর্ততা / আনমনা ভাব

Ex. Sentence: I could see she was in a brown study.
Bangla Meaning: আমি দেখতে পাচ্ছিলাম সে খুব আনমনা হয়ে আছে

Source: Lecture.