Final answer:
The presence of many buyers and sellers in a competitive market is primarily responsible for keeping prices low as it prevents any single entity from controlling the market price.
Step-by-step explanation:
The characteristic of a competitive market mainly responsible for ensuring that prices will be kept low is B) Many buyers and sellers. In a competitive market, the presence of many buyers and sellers means that no single buyer or seller has significant control over the price. This ensures that the prices of goods and services reflect the forces of supply and demand. Monopolies, on the other hand, can set higher prices because they face no competition due to barriers to entry like legal restrictions, technological challenges, or market control over resources. Government regulation, while it can affect prices, is not the main characteristic that keeps prices low in a competitive market.