Paragraph: Descriptive:: Letter:

• Descriptive paragraph হচ্ছে এক ধরনের paragraph.
- তেমনি Memorandum হচ্ছে একধরনের letter.

• There are four types of paragraph writing.
- যথা -narrative, descriptive, expository, and persuasive.
- Descriptive paragraph: A descriptive paragraph is a focused and detail-rich account of a specific topic. 
- Paragraphs in this style often have a concrete focus - the sound of a waterfall, the stench of a skunk’s spray - but can also convey something abstract, such as an emotion or a memory. 

 • Letter দুই প্রকার।
যথা -
i) Informal Letter : যা ব্যক্তিগতভাবে আত্মীয়স্বজন বা বন্ধুবান্ধবের কাছে পাঠানো হয়।
ii) Formal Letter: Official বা Business সংক্রান্ত চিঠিপত্র।
যেমন - Application, Memorandum ইত্যাদি।
- সুতরাং,
• “Memorandum” is one kind of - Formal Letter.
1.  a short note written to help one remember something or remind one to do something/  a record of events or observations, esp. one for future use. 
2.  an informal written communication, as from one department to another in an office.

• অন্য অপশন গুলোর মধ্যে - 
• Corrigendum - A thing to be corrected, typically an error in a printed book.
• Linkers and Cohesion are used in a paragraph.

Source: Collins Dictionary.