A good tip to keep your computer virus-free is to:

The correct choice emphasizes the importance of being cautious with executable or script files, which are often associated with malware. Files with extensions like .exe (executable) and .vbs (VBScript) can run programs or scripts that may contain harmful code, making them a common method for spreading viruses. By avoiding these files, users significantly reduce the risk of inadvertently downloading and executing malicious software on their computers.

The other options, while offering advice, do not provide the same level of practical security. Avoiding antivirus software based on origin does not ensure safety, as many reputable programs are globally developed. Consulting a physician about email safety is unrelated to computer virus protection and reflects a misunderstanding of the context. Proofreading messages, while good practice for clarity and professionalism, does not impact the computer's virus status or security. Thus, endorsing caution with executable files is the most effective approach to maintaining a virus-free computer.