. . . . . . . . fourth boy in . . . . . . . . third row, is my son.

In the given sentence, the speaker is referring to a boy who is standing in a particular position in a particular row i.e. "the fourth position in the third row". Hence, the noun 'fourth boy' and 'third row' should be preceded by the definite article 'the'. Option A is the only option which indicates the use of the article 'the' in both the blanks and is the correct option.