I am not sure but I find "I see" or "I understand" is little bit formal. I am not sure close friends say each other "I see" a lot. I could be wrong, though. It sounds more like a doctor can say it when you talk about your pains.
Here is my scenario
A: Why did you get to class late today?
B: Well, my phone was out of charge (while I was sleeping). So it didn't go off and I missed the train.
C: I see, ok, right or ?
(Could you please rephrase B's the first sentence because I am no sure it is colloquial)
Best Answer
"I see." is informal, and is often used by friends talking to each other.
The original poster is correct that "I understand" is more formal than "I see", and that both "I understand" and "I see" are often used by doctors who are listening to patients.
If you are not being sarcastic, you could say:
The following options might be interpreted as being sarcastic:
"I grok" would not be appropriate for your example. "Grokking" is even less formal than "seeing", and describes a profound level of understanding.