Final answer:
Active continental margins are typically sites of significant tectonic activity and are located on the west coasts of both North and South America. The east coasts of the United States and South America along with the coastlines around Africa represent passive margins.
Step-by-step explanation:
Active continental margins are regions where a continental plate meets an oceanic plate, often characterized by tectonic activity such as earthquakes and volcanism. These active margins can be contrasted with passive margins, which are more stable and lack such tectonic activity. Examining the options given in the question:
- (a) Along the east coast of the United States - represents a passive margin, as it lacks significant tectonic activity and is where the sediments have accumulated since the eastern edge of the continent became tectonically inactive.
- (b) Along the east coast of South America - also has a passive margin on the Atlantic side for similar reasons.
- (c) On the west coasts of both North and South America - these coasts are indeed active margins, with the west coast of North America having the San Andreas fault and the west coast of South America having the Andes Mountains, both of which are results of tectonic activities.
- (d) All around Africa - Africa is mostly surrounded by passive margins, not active ones.
Therefore, the correct answer is (c) on the west coasts of both North and South America where the active continental margins are located.