Answer:
¿Cuántos
Step-by-step explanation:
It is ¿Cuántos because it doesn't make sense to say ¿Quién arboles hay? Hay seis árboles which means Who trees are there? There are six trees. So we would have to try ¿Qué arboles hay? Hay seis árboles which means What trees are there? There are six trees . So then again we would try ¿Cuántos arboles hay? Hay seis árboles. which means How many trees are there? There are six trees.