Final answer:
The word that best completes the sentence 'está en la boca' is 'la lengua'.
Step-by-step explanation:
The Spanish word that best completes the sentence 'está en la boca' is la lengua. 'Está en la boca' translates to 'It is in the mouth' and the word for 'tongue' in Spanish is 'la lengua'.
Learn more about Spanish vocabulary