Final answer:
The purchase of Alaska was initially controversial due to concerns about its economic viability and purchase cost, but the discovery of gold and oil (option A) changed public opinion by revealing the territory's valuable resources.
Step-by-step explanation:
The purchase of Alaska in 1867 was initially controversial because there were concerns about the economic viability of Alaska and the perceived high purchase price of $7.2 million. The region was mockingly referred to as "Seward's Folly" and "Seward's Icebox", as many Americans did not see the value in acquiring a remote and seemingly barren territory.
Public opinion about the purchase changed dramatically with the discovery of valuable resources, most notably gold, which triggered the Klondike Gold Rush, and later, significant oil reserves. The discovery of these resources helped justify the purchase and demonstrated that Alaska was indeed economically beneficial to the United States.
In summary, the initial controversy stemmed from doubts about Alaska's value, but was later transformed by the revelation of its natural resources, especially after valuable minerals and oil were found, underscoring Alaska's potential and importance.