Which of the following conclusions do u. s. attitudes toward the panama canal during the 20th century support?

a. peoples points of veiw can change to suit changes in historical context.
b. biases prevent most people from participating in a changing historical context.
c. contextualization of a historical event should not be changed once it has been made.
d. historical context can only be determined long after an event has passed.

A.  People's point of view can change to suit changes in historical context.

Allow me to add some explanation about historical context and how that works.

When the Panama Canal was built in the early 1900s, the historical and political context was one of imperialism. President Theodore Roosevelt of the United States used military pressure and warships to force Columbia to recognize Panama's independence, and then arranged with Panama to cede a strip of land to the United States (for $10 million) so that the US could build the canal and control the canal zone for commercial purposes.
By the late 20th century, the historical and political context was much different, and imperialistic control of overseas territories was no longer in vogue.  In 1977, US President Jimmy Carter signed a treaty with Panama which agreed to give over control of the canal to Panama by the end of the century, with transfer of control taking place on December 31, 1999.   
Part of the reason for willingness to make that transfer was that the Panama Canal had become expensive for the United States to maintain and operate -- so that part of the historical context plays in also.  Since Panama took over the canal, they have actually done very well financially with it.  You could check out a good book on the subject, The Big Ditch, by Noel Maurer and Carlos Yu, published by Princeton University Press in 2010.

Democracy and darwinism would be my answer.