Which scenario exhibits the “social proof” principle of human influence?

Which scenario exhibits the “social proof” principle of human influence?

• A user receives a phone call from someone claiming to be from the IT department who needs to verify user credentials.
• A user visits a web site offering a free gift if the user provides contact information.
• A user opens an email advertising a free trial on a website that is only available for a few hours.
• A user completes a survey requesting personal information after seeing co-workers completing the survey.