Adjectives of number are the adjectives that are used to depict either the number of nouns or their position or place in a certain order.
Option A: Definite numerical adjective: These adjectives specify a definite number like one, sixth, fifth, double, two etc. This is exactly the case in the sentence.
Option B: They give indefinite amount like some, few, many, all, no, several etc but are used for countable nouns.
Option C: Examples of distributive adjectives are each, every, neither, either, etc.
Option D: Demonstrative adjectives are adjectives that are used to modify a noun so that we know which specific person, place, or thing is mentioned.
Option A is the correct answer.