The correct answer is A. in.
The preposition “in” is used to indicate a location or position within a place. In this case, the custom is said to have originated in an East European country. This means that the custom began in that country and is still practiced there today.
The other options are not as accurate. The preposition “from” is used to indicate a source or origin. In this case, the custom is not said to have come from an East European country, but rather to have originated there. The preposition “by” is used to indicate a means or agent. In this case, the custom is not said to have been created by an East European country, but rather to have originated there. The preposition “with” is used to indicate a connection or association. In this case, the custom is not said to be associated with an East European country, but rather to have originated there.