Which form of advertising is designed primarily to encourage viewers to support social causes?

The correct choice is public service advertising (PSA) because this form of advertising is specifically aimed at promoting social causes and encouraging positive social behavior. PSAs often highlight issues such as health, safety, environment, or social justice, seeking to raise awareness and motivate viewers to take action or change their attitudes.

Unlike other forms of advertising, such as persuasive advertising that focuses on convincing consumers to buy a product or commercial advertising that promotes specific brands or services for business gain, PSAs prioritize societal benefits over commercial interests. Informative advertising, while providing information about a product or service, does not usually center on social impact or advocacy but rather aims to educate the audience on features and benefits. Thus, PSAs serve an essential role in fostering community engagement and support for various causes, making it the best fit for the question.