In pavlov's experiments, the meat powder (food) placed on the dog's tongue was the

The best answer would be:

Unconditioned stimulus

In Pavlov's experiment on conditioning the food was the unconditioned stimulus that triggers a reflex or natural behavior. Think of it like something that makes you do something automatically.

For example, the food. When the dog smells food, it salivates. This is unconditioned because without teaching the dog, it automatically responds by salivating.