Final answer:
The Israelites' reaction to Moses being up on the mountain for 40 days was building a golden calf.
Step-by-step explanation:
The Israelites' reaction to Moses being up on the mountain for 40 days was C. Building a golden calf.
In the story from the book of Exodus, when Moses returns, he finds that the Israelites have constructed a golden calf to worship. This was considered a betrayal of the one and only God who had just delivered them from slavery in Egypt.
This incident led to Moses throwing down and smashing the stone tablets that had the terms of the covenant written by God himself.