In his free time Cris enjoys...the Net. * a. surfing b. chatting c. hanging out

The most fitting answer, considering the context of enjoying the internet, would be:

a. surfing

This is commonly referred to as "surfing the net," which implies browsing or exploring the internet. However, if Cris specifically enjoys chatting or socializing online, either b or c could also be appropriate depending on his preferences. If I had to choose the most general answer, it would be "a. surfing."