Your answer would be caballete.
La pintura acuarela es muy clara y en el lienzo se pueden mezclar muchos colores para reflejar la luz. Es muy importante tener un ________ de madera para sostener el lienzo.
Translated
Watercolor paint is very light and many colors can be mixed on the canvas to reflect light. It is very important to have a ________ of wood to support the canvas.
Caballete translates to easel, which is usually made out of wood to support the canvas.
I hope this helped!