The product life cycle stage in which there is a rapid sales increase, other firms have begun to market competing products, and lower unit costs and an overall tendency toward increased profit prevail is

Growth

Explanation:

The stage of growth is defined as the period or year during which the product in due course and increasingly acquire or gains the acceptance among the industry, customers and wider general public.

So, the growth stage is that stage where there is rapid increase in sales and other firms started to market the competing the products and also lower the unit costs.

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