If you use "today's", "contemporary" becomes redundant.
What discovery in last 100 years has been most beneficial for people in your country?
The prompt asks you about the most beneficial discovery
in the last 100 years. However, your essay is about the Internet, which is an invention.
As far as I'm concerned, their meanings are not similar:
- Discovery is finding out or figuring out something that already exists.
- Invention is using objects that preexist to create something new that is first of its kind.