Which of the following Trojans is used by attackers to modify the Web browser settings?

Win32/FlyStudio is a backdoor trojan that provides remote access capabilities but is not specifically associated with modifying web browser settings.

Trojan.Lodear is primarily a dropper that downloads and installs additional malicious files to disk; it does not specifically target browser configuration.

WMA/TrojanDownloader.GetCodec spreads as a Windows Media Audio file that prompts users to install a fake codec update. Once the malicious payload executes, it alters browser configuration values such as the home page and default search provider, redirecting victims to attacker-controlled sites and enabling further compromise.

Win32/Pacex.Gen is a generic heuristic detection for obfuscated or packed malware families and is not specifically known for browser hijacking behavior.