Step 1: Understanding the Concept:
The question asks about the primary economic activity and source of employment for the population of the state of Bihar.
Step 2: Detailed Explanation:
The economy of Bihar is predominantly based on agriculture. It is an agrarian state where a very large percentage of the population (around 80%) depends on agriculture and allied activities for their livelihood. The fertile Gangetic plains provide a suitable environment for farming. While industry and trade exist, their contribution to the state's economy and employment is significantly smaller compared to agriculture.
Step 3: Final Answer:
The main source of livelihood in Bihar is Agriculture.