D. The images are multi-colored to show Hindu acceptance of all races.
It cannot be painted on canvas since it may flake off. The painting surface must also be prepared with chalk gesso. You must paint thinly and each layer must cure. It also dries quickly so a lot of skill is required to get smooth results.
When you are talking to someone and you want a certain phrase you said to stand out, you emphasis your words. That is pretty much the same case here. A is incorrect because that is capturing a model of some sort, B is incorrect because that would be an equal blend of all colors together, and D is incorrect because you are not uniting the colors, you want the elements to stand out. Emphasis is achieved usually by using mostly dark colors then a splash of something such as neon pink or yellow.
Answer is A - airbrushing
