A business wants to forecast reach for a campaign with a specific audience and budget. Which tool should the advertiser use to perform this task?

The most appropriate tool for forecasting reach for a campaign with a specific audience and budget is the audience definition tool. This tool allows advertisers to input specific parameters regarding their target audience, including demographics, interests, and behaviors, as well as the budget allocated for the campaign. By doing so, advertisers can gain an understanding of the potential size of their audience and the estimated reach based on their budget and audience settings.

Using the audience definition tool is particularly beneficial because it provides a preview of how many people can potentially be reached within the defined audience parameters. This information is crucial for planning and optimizing ad campaigns, ensuring that businesses can make informed decisions about their advertising strategies and expectations regarding engagement.

Other tools like Facebook Insights provide valuable analytics about past campaign performance and insights, but they do not specifically forecast future reach based on budget and audience parameters. The Ad Preview tool allows advertisers to see how their ads will look in various placements but does not address forecasting reach. The Reach Estimator, while it appears related to reach, is not specified as the best tool for audience and budget forecasting within the context of the question. Thus, audience definition is clearly the most suitable option for the task at hand.