The correct preposition here is "from." This is because "from" indicates that the speaker is from a specific place, in this case Brazil. The preposition "at" is not correct because it is used to indicate a specific location or address. For example, "I am at the gym" would be correct because the gym is a specific location. The preposition "on" is not correct because it is not typically used to indicate place of origin. It is used to indicate a location on a surface, or a location in a sequence of events.