Fernando: Tuve que comprar unas sillas.
Víctor: ¿Dónde ______ compraste?
Translation:
Fernando: I had to buy some chairs.
Victor: Where [blank] you buy?
To find the correct direct object pronoun, identify the object. Our object is unas sillas, which is feminine plural.
The options
A. los - the ~masc. plural
B. la - the ~fem. singular
C. las - the ~fem. plural
D. lo - the ~masc. singular
The gender and grammatical number between the object and the object pronoun must agree, thus the correct answer is (C).
Answer:
C. las