Right Answer:
el
This is an interrogative sentence, where the subject (noun) is masculine singular.
In Spanish grammar the article is a word that clearly indicates the gender and the number of the noun. Its function is to modify the noun it introduces.
The articles are as follows:
Variable Articles:
Defined Articles:
Singular Masculine: el
Singular Femenine: la
Plural Masculine: los
Plural Femenine: las
Indefinite Articles:
Singular Masculine: un/uno
Singular Femenine: una
Plural Masculine: unos
Plural Femenine: unas
Invariable Articles:
Neutral: Lo
Contractions: al (a+el) and del (de+el)
Now, going back to the sentence, and remembering the noun muchacho (boy) is masculine singular. The article that fits here is the Defined Article in Singular masculine el
Therefore, the complete questions is:
¿Quién es el muchacho rubio?
Who is the blond boy?