Which became the world’s largest communications network in the early twenty-first century?

The Internet, is the correct answer.

Explanation:

The Internet is the global arrangement of interconnected computer networks that employ the Internet protocol suite to link devices universally. It is a chain of networks that comprised consists of public, private, business, academic, and government networks of regional to the global range, connected by an extensive array of electronic, visual and wireless networking technologies. The internet has become the largest communication network in the world in the early twenty-first century.

a drastic rise in oil prices ended decades of postwar.