The recommended format for implementing schema markup is JSON-LD.

• Google Tag Manager
• HTML
• JSON-LD
• None of the above

The correct answer is: JSON-LD

Explanation: JSON-LD stands for JavaScript Object Notation for Linked Data. It is a script that can be embedded within a web page to convey structured data to search engines. While there are various formats available for adding structured data, JSON-LD is the preferred choice by Google. Its simplicity and ease of placement within the web page’s code make it a less complex method compared to other available formats.

Source: HubSpot Lesson: Optimizing Your Website for Rich Results