Answer:
Firm is exploiting Economies of Scale
Step-by-step explanation:
Economies of Scale refers to the cost saving advantages ( cost reduction), that firms experience due to increasing scale of production level.
Returns to scale just signifies that : when all inputs increase in same proportion ; the output increase with - same proportion (constant returns), or lesser proportion (decreasing returns), or higher proportion (increasing returns).
In the given case : the inputs have not increased, the output has doubled. So, this illustrates Economies of scale. As, the firm has experienced cost advantages in production, simultaneous to increasing (doubling) production level.