Formal:
* সেটা অবশ্যই সত্য। (Seta oboshshoi satya.) - This is the most literal translation and is very formal.
* আপনার কথা সত্য। (Aponar kotha satya.) - This means "Your words are true" and is a more polite way to agree with someone.
Informal:
* ঠিকই তো। (Thikই to.) - This is a very casual way to agree and means "That's right."
* হ্যাঁ, ঠিক কথা। (Haa, thik kotha.) - This means "Yes, that's right."
* সত্যিই বলছেন। (Sotyoi bolchhen.) - This literally means "You are really saying [it]" and implies that the statement is very true.
* একদম ঠিক। (Ekdom thik.) - This means "Absolutely right."
Other options:
* সত্যি বলতে কি, ... (Sotyoi bolte ki, ...) - This is a phrase used before saying something that is very true, and means "To tell the truth...".
* সেটা অবশ্যই। (Seta oboshshoi.) - This is another way to say "That's definitely true."
The best way to choose the right expression depends on the situation and your relationship with the person you are speaking to.