A Custom Audience is the most effective option for reaching existing customers. This targeting tool allows advertisers to create an audience from their own sources, such as a customer list (emails, phone numbers), website traffic data from the Meta Pixel, or mobile app activity. By uploading this data, a business can directly target ads to people who have already purchased from them or interacted with their brand, making it the ideal method for engaging a current customer base.

A. Lookalike Audience: This targets new people who share similar characteristics with your existing customers, which is for prospecting, not reaching the customers themselves.

C. Conversion audience: This is not a distinct audience category. It's a strategic use of other audience types, typically a Custom Audience of past converters.

D. Interest-based audience: This targets people based on their general interests and behaviors on Meta platforms, not their specific history with your business.

1. Meta Business Help Center, "About Custom Audiences." This document states, "A Custom Audience is a type of ad targeting that lets you find your existing audiences among people who are on Facebook... You can use sources like customer lists, website or app traffic, or engagement on Facebook, to create Custom Audiences of people who already know your business." (Meta Business Help Center, Article ID: aBu33372).

2. Meta Business Help Center, "About Lookalike Audiences." This source clarifies the purpose of Lookalikes: "Lookalike Audiences are a way to reach new people who are likely to be interested in your business because they're similar to your best existing customers." (Meta Business Help Center, Article ID: aBu33373).

3. Meta Blueprint, "Introduction to Meta Ad Audiences" Course. In the module on "Audience Types," the course material explicitly defines Custom Audiences as the tool to "re-engage people who have already interacted with your business" and distinguishes this from Core Audiences (interest-based) and Lookalike Audiences, which are used for reaching new potential customers. (Meta Blueprint, Course 431-01).