¿quién es muchacho rubio?
a.
el
c.
los
b.
la
d.
las

Right

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:

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

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:

Who is the blond boy?