The correct answer is: [B]: " unas " .
→ that is; " Unas marcadoras."
_____
Note: The question asks about the particular Spanish word—marcadoras—and which form of the indefinite article—
Note: The term "indefinite article" refers to:
["some"] (plural) ; or, ["a(n)"] (singular)} would be used.
We are given the following answer choices:
[A]: ' un ' ;
[B]: ' unas ' ;
[C]: ' unos ' ;
[D]: ' una ' .
_____
Note that these answer choices constitute all the possible indefinite articles in the Spanish language.
Note that in Spanish, the word: "marcadoras"—is a feminine, plural noun. Furthermore, the Spanish word: "marcadora"—is the feminine, single noun version. This single feminine Spanish noun, along with the definite article, would be: la marcadora.
_____
Note that for Spanish—{Note: exception explained at end of this answer.}:
As a general rule: {Note: Exception explained at end of this answer.}:
Definite article Indefinite article
["the"] ["a(n}"—singular]; or;
["some"]—plural]:
Single noun, masculine: el un
Single noun, feminine: la una
Plural noun, masculine: los unos
{→ or: [group of at least
one masculine;
and one or more feminine] ;
→ take the plural noun, masculine: los unos
⇒ i.e. {"same as above"}.
__________________________________________________
Plural noun, feminine: las unos
Since the Spanish word: "marcadoras" —is a plural, feminine noun, the indefinite form would be unas marcadas ;
→ which corresponds to Answer choice: [B]: " unas ".
_____
Note: This should not be confused with the Spanish word:
→ el marcador ; [single noun, masculine];
in which the plural form would be:
→ los marcadores ; and the indefinite pronouns for both the single noun form and plural noun forms, respectively, would be "un" ; and "unos".
_____
Other note: One expection in the Spanish language:
For Spanish words in that begin with the Spanish a as the first syllable, the single, noun form, for the indefinite article is "el" , and the single noun, indefinite article is "un" —by convention—since otherwise the pronunciation would be difficult to comprehend}.
Examples: El agua as divertida. La usamos para cocinar.
water—el agua (feminine);
Africa—el África (feminine):
soul—el alama (feminine).
_____
Hope this is helpful to you;
Best wishes to you in your academic endeavors!
_____