Final answer:
The Spanish preposition that correctly completes the sentence 'The pencil is inside my backpack' is 'dentro de'.
Step-by-step explanation:
The Spanish preposition that correctly completes the sentence 'The pencil is inside my backpack' is dentro de. So, the correct sentence would be 'El lápiz está dentro de mi mochila.' The preposition 'dentro de' means 'inside'.
Learn more about Spanish prepositions