Final answer:
Marketing research became widespread in the early 20th century, aligning with technological advancements and the growing influence of corporations which fostered new marketing methods.
Step-by-step explanation:
Marketing research became widespread in the early 20th century (option b). This was a time characterized by major technological advancements and a significant increase in corporation influence. The adoption of new technologies and the rise of corporations facilitated the expansion of marketing techniques and the ability to reach a wider audience. Moreover, the development of commercial radio broadcasting and increased automobile accessibility contributed to the advent of a popular culture and an American mass identity. Thus, modern marketing tactics began to flourish, and by the turn of the century, marketing had become a multi-million dollar industry.