Questions: Ellos caminan a la biblioteca.

The answer is "adónde": Ellos caminan a la biblioteca.

Explanation:

1. "Adónde" is used to ask about the destination or direction of movement. The question would be "¿Adónde caminan ellos?" which translates to "Where are they walking to?" The answer is "Ellos caminan a la biblioteca" (They are walking to the library).

Here is a brief explanation of why the other options are incorrect in this context:

• "Cuántos" asks about quantity (e.g., "How many?").
• "Por qué" asks for a reason (e.g., "Why?").
• "Quiénes" asks about people in the plural form (e.g., "Who?").
• "Cuáles" asks about choices or options (e.g., "Which ones?").
• "De dónde" asks about origin (e.g., "From where?").
• "Qué" asks about things or actions (e.g., "What?").
• "Cuándo" asks about time (e.g., "When?").
• "Dónde" asks about location (e.g., "Where?").
• "Quién" asks about a person in the singular form (e.g., "Who?").