Raymond Vernon noticed that in the 1960s, the wealth and size of the U. S. market was a natural incentive to develop new consumer products. What theory did he propose based on this fact?

The correct answer is letter "D": product life-cycle.

Explanation:

American economist Raymond Vernon (1913-1999) introduced the "International Product Life-Cycle" in 1966 to explain how goods and services are marketed, continue rising, to be forgotten later by consumers. The cycle consists of four (4) stages, according to Vernon:

The introduction stage: successful product development and marketing efforts.  The growth stage: increase in sales, decrease in production costs, and increase in revenue as a result of the product being noticed by consumers.  The maturity stage: broad recognition of the product, which brings competition and drives the company's efforts to remain stable.  The decline stage: saturation of the market, leading to a decline in sales and unpopularity of the product.