Formal:
* Eu compreendo. (Most literal translation, more formal)
* Eu entendo. (Also formal, but more common)
Informal:
* Eu entendi. (More colloquial, means "I understood," but can be used in place of "I understand")
* Entendi. (Even more informal, just the verb "understood" by itself)
* Sim, senhor/senhora. (Yes, sir/madam, used as a confirmation of understanding)
* Compreendi. (Less common, but also formal and emphasizes understanding)
Other options:
* Eu captei. (Literally "I caught," implies understanding a message)
* Eu peguei. (Similar to "I caught," used in informal contexts)
The best choice will depend on who you are talking to and the situation. For example, you would use "Eu compreendo" in a professional setting, while "Entendi" would be appropriate with friends or family.