The correct answer is: A. one.
The word “one” is used to refer to a person or thing that is not specifically identified. In this case, the speaker is not identifying the European they met, so they use the word “one”.
The word “a” is used to refer to a person or thing that is not specifically identified, but is considered to be one of a group. In this case, the speaker could have used the word “a” if they had been referring to any European, but they are specifically referring to one European, so they use the word “one”.
The word “an” is used to refer to a person or thing that is not specifically identified, but is considered to be the first of a group. In this case, the speaker could have used the word “an” if they had been referring to any European, but they are specifically referring to one European, so they use the word “one”.
The word “the” is used to refer to a person or thing that is specifically identified. In this case, the speaker is not identifying the European they met, so they do not use the word “the”.