Final answer:
The main advantage of using sales force opinions in demand forecasting is their awareness of customers' future plans, which aids in making accurate predictions.
Step-by-step explanation:
The advantage of using the opinions of a sales force to develop a demand forecast is that salespeople often have direct contact with and insight into customers' future plans which can inform predictions about future sales. While salespeople may have biases or be influenced by recent events, the fourth option is most accurate because of their direct relationship with customers. They are positioned to know about upcoming projects or expansions that could affect demand for products and services, making the sales staff's awareness an asset in forecasting.