Final answer:
To troubleshoot a Windows computer's network connectivity issues, use the Network Troubleshooter, as it is designed to diagnose and potentially fix network problems.
Step-by-step explanation:
To troubleshoot a Windows computer that isn't connecting to a network, the correct utility to use would be C) Network Troubleshooter. This utility is specifically designed to diagnose and resolve various network-related issues. It can help identify the cause of the connectivity problem and often can fix the issue automatically. Disk Cleanup is used for freeing up space on your hard drive, Disk Defragmenter optimizes file organization for quicker access, and System Restore can revert your computer's settings to an earlier point in time, but these utilities are not meant for network troubleshooting.