Final answer:
The maturity stage of the product life cycle is when sales growth slows, competition intensifies, and profit margins decrease over a prolonged period of time.
Step-by-step explanation:
The stage of the product life cycle that stretches over a relatively long period of time during which the profit margin decreases is the maturity stage. During this phase, sales growth slows down, and the product faces intense competition, which often leads to price reductions and higher marketing expenses to maintain market share. As a result, profit margins typically decrease. This stage contrasts with the introduction, which is when a product is first launched, the growth stage, where sales rapidly increase, and the decline stage, where sales and profits fall, often leading to the discontinuation of the product.