To read between the lines’ means —

1. to read carefully
2. to read some lines only
3. to read carefully to find out any hidden meaning
4. to read quickly to save time.

Answer: to read carefully to find out any hidden meaning

Explanation: ‘Read between the lines’ idiom টির to read carefully to find out any hidden meaning । (প্রকাশিত হয়নি এমন অর্থ অন্বেষণ বা আবিষ্কার করা)।