The Sales Era resulted from:

an effort to personalize sales

Right! The era of sales was resulted due to Too Much Competition ....

Option D is the correct one

^^^^^^^^^^^^^^^^^^^^^^^^^D^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

D. too much competition. After the industrial revolution, competition grew and focus turned to selling. Therefore marketing, sales and branding became an important pillar as outputs surpassed demand, and firms competed for customers.

B............................................

too much competitions.....:)

B. an effort to personalize sales

With good marketing. 

B. an effort to personalize sales......................................

An effort to personalize sales (B) because it were too much competitors (D)

C. the firms realizing they could produce more goods