The correct answer is A. Check for malicious applications, because excessive data usage, device overheating, and rapid battery drain are classic indicators of mobile malware or spyware running in the background.
According to the Quentin Docter - CompTIA A+ Complete Study Guide, malicious applications can continuously transmit data, mine cryptocurrency, or monitor user activity, all of which cause increased network usage and battery consumption-even when the device appears idle.
The Travis Everett & Andrew Hutz - All-in-One Exam Guide emphasizes that the first step in mobile troubleshooting is identifying whether unauthorized or malicious apps are present. Reviewing installed applications and running a mobile security scan can quickly confirm the root cause.
The Mike Meyers / Mark Soper Lab Manual explains that disabling networks or enabling airplane mode only masks symptoms temporarily and does not remove malware. Factory resets are effective but should be used after confirming malware or if other remediation steps fail.
Because the symptoms strongly suggest malicious background activity, the most appropriate first action is to check for malicious applications, making A the correct answer.