Final answer:
Marketers can expect the highest frequency of adoption for a given product when it reaches the early majority, according to the diffusion of innovation curve. The correct option is 4.
Step-by-step explanation:
According to the diffusion of innovation curve developed by sociologist Everett Rogers in 1962, marketers can expect to see the highest frequency of adoption for a given product when the product reaches the early majority.
The diffusion of innovations theory suggests that after innovators and early adopters start using the product, the early majority will begin to adopt it, which is generally when the adoption rate significantly increases. This stage comes after the product has been on the market for some time and has begun to gain traction among consumers.