When creating a Lookalike Audience, an advertiser must make two fundamental choices. First, they must select a source audience. This is a pre-existing Custom Audience (e.g., from a customer list, website visitors, or app users) that serves as the seed for Facebook's algorithm to find new people with similar characteristics. Second, the advertiser must define the audience size. This is set using a percentage slider (from 1% to 10%) of the total population in the chosen location(s). A smaller percentage creates a more precise but smaller audience, while a larger percentage increases reach at the cost of similarity.

B. Demographics: Demographics are not manually selected during Lookalike creation; the algorithm infers them from the source audience to find a match.

D. Interests: Similar to demographics, interests are identified by the system based on the source audience, not chosen as an input by the advertiser.

1. Meta Business Help Center, "About Lookalike Audiences." This document states, "When you create a Lookalike Audience, you choose a source audience... You can also choose your desired audience size during the creation process." This directly confirms the necessity of selecting a source and size.

Source: Meta for Business, Help Center, Article: "About Lookalike Audiences".

2. Meta Business Help Center, "Create a Lookalike Audience." The step-by-step guide for creating a Lookalike Audience explicitly lists the required actions:

Step 3: "Choose your source audience."

Step 5: "Use the slider to choose your audience size."

Source: Meta for Business, Help Center, Article: "Create a Lookalike Audience".