Saber is knowing how to play the guitar, knowing how to do something, knowing a language, having knowledge about something. Conocer is more like being aware of something, not necessarily having knowledge about it. If you say "conozco la guitarra" it means you know the instrument, you know what a guitar is, but it doesn't imply you can play it. You use conocer in the context of knowing a place, knowing a person, and so on. So you would say "¿tú sabes tocar la guitarra?"
sabes
Saber is to know of something or how to use it Conocer refers to knowing a person
The correct verb to fill in the blank in the question is "saber" because it relates to knowing how to do something, specifically playing the guitar. Thus, the complete question is "¿Tú sabes tocar la guitarra?"
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