An app builder installed a custom Lightning component from AppExchange.

The correct answer is B. Edit a record page using Lightning App Builder > Drag the component onto the page.

A custom Lightning component installed from AppExchange is configured on a Lightning record page through Lightning App Builder. Salesforce documentation states that Lightning App Builder supports custom Lightning components, including Aura and Lightning web components, and record pages are edited as Lightning pages.

The Page Layout editor is used for fields, buttons, related lists, and layout behavior, not for placing Lightning components on a Lightning record page. App Manager manages apps and utility bars, but the page canvas configuration is done in Lightning App Builder.