The fundamental source of monopoly power is b) barriers to entry.
Barriers to entry refer to the obstacles that prevent new competitors from easily entering a market. These can include high startup costs, strict regulatory requirements, and strong brand loyalty towards existing companies. When barriers are high, existing firms can maintain their market dominance and set prices without the pressure of new entrants who could potentially drive prices down. This lack of competition allows monopolies to exert significant control over their market, leading to higher prices and reduced output compared to a competitive market. Thus, while factors like profits and unique products play a role, it is the barriers to entry that primarily sustain monopoly power in the long run.