The correct answer is A. ninth.
The boy you met yesterday is in class ninth. This is because the word “ninth” is an ordinal number, which is used to indicate the position of something in a series. In this case, the boy is in the ninth position in his class.
The other options are incorrect because they are not ordinal numbers. The word “the” is a definite article, which is used to identify a specific person or thing. The word “nine” is a cardinal number, which is used to indicate the quantity of something. The word “nine” is not an ordinal number because it does not indicate the position of something in a series.