Online gaming activity is the most likely reason for the increase in network traffic because it consumes a lot of bandwidth and data. Online games often require constant communication with servers and other players, which can generate a lot of network traffic. Unnecessary OEM software, lack of antivirus, and firewalls are not configured are less likely to cause such a significant increase in network traffic, as they are either one-time or periodic processes that do not require as much data transfer as online gaming. Reference:
CompTIA IT Fundamentals FC0-U61 Certification Study Guide, page 101, section 2.4: "Online gaming can consume a lot of bandwidth and data." CompTIA ITF+ Practice Test, question 221: "Online gaming activity is the correct answer."