The synonym of DECLINE is -

• The synonym of 'Decline' is - Diminish.

• Decline (verb)
- English Meaning: to become smaller, fewer, weaker, etc.
- Bangla Meaning: কমা; হ্রাস পাওয়া; পতন হওয়া।

• Given options:
ক) Diminish
- English Meaning: to become smaller, weaker, etc.; to make something become smaller, weaker, etc.
- Bangla Meaning: হ্রাস করা; হ্রাসপ্রাপ্ত হওয়া।

খ) Increase
- English Meaning: to become or to make something greater in amount, number, value, etc.
- Bangla Meaning: বৃদ্ধি করা; বাড়ানো।

গ) Abandon
- English Meaning: to leave somebody, especially somebody you are responsible for, with no intention of returning.
- Bangla Meaning: পরিত্যাগ করা; ছেড়ে দেওয়া।

ঘ) Postpone
- English Meaning: to arrange for an event, etc. to take place at a later time or date.
- Bangla Meaning: [পাস্‌পোন্] (verb transitive) স্থগিত/মুলতবি রাখা/করা: postpone a meeting.

• সুতরাং, অপশনের অর্থ বিবেচনা করে দেখা যায় যে, The synonym of 'Decline' is - Diminish.

Source:
1. Accessible Dictionary by Bangla Academy.
2. Oxford Learner’s Dictionary.