Final answer:
The sentence should be completed with the word 'Cuántas' to ask about the quantity of something.
Step-by-step explanation:
The sentence should be completed with the word 'Cuántas'. The correct sentence should be 'Cuántas pelotas hay?'
The word 'Cuántas' is the interrogative form of 'cuánto' which means 'how many' in English. It is used to ask about the quantity of something.
For example, if you want to ask how many balls there are, you would say 'Cuántas pelotas hay?'
Learn more about Spanish grammar