Final Answer:
A competing operating system to Windows Phone 7 is Android. Option 2 is the answer.
Step-by-step explanation:
Windows Phone 7 was Microsoft's mobile operating system introduced as a competitor to other smartphone platforms. Among the options provided, Android is the primary operating system that competes with Windows Phone 7. iOS is the operating system developed by Apple for its devices, BlackBerry was a proprietary operating system used by BlackBerry smartphones (no longer actively developed), and Symbian, though historically significant, lost prominence in the smartphone market.
Android, being an open-source operating system used by various manufacturers, emerged as a major competitor to Windows Phone 7 due to its widespread adoption across different device manufacturers.
Option 2 is the answer.