The author draws a clear connection between Earth Day and corporations showing a significant shift that occurred around 1990. Initially, when Earth Day started in 1970, companies were largely uninterested in their environmental image. However, by the 1990s, corporations began to see Earth Day as an opportunity to showcase their "green" credentials and improve their public reputation. This shift was part of a broader transformation due to changes in industry focus and a more competitive market where brand image became increasingly important.
Specifically, the author notes that in 1990, Earth Day organizers aimed to "steer millions of investors and consumers toward companies judged to be most sensitive to environmental concerns." Business executives even joined environmentalists on the Earth Day board. This corporate involvement sometimes involved "greenwashing," where companies project an environmentally responsible image to appeal to consumers without necessarily making substantive environmental improvements. For example, BP rebranded itself as "Beyond Petroleum" to highlight a greener image.
Thus, the correct answer to the question about the connection the author draws between Earth Day and corporations is:
C. Companies eventually started using Earth day to show how green they are.