Step-by-step explanation:
The sentence that shows the correct use of the serial comma is:
c. wanted to guard against foreign intervention, provide economic security, create a more efficient national government, and improve political unity for the nation.
In this sentence, the serial comma (also known as the Oxford comma) is placed after each item in the list, including before the conjunction "and" that precedes the last item. This is the recommended usage in standard English writing, as it helps to clarify the separation between items in a list.