Which of the following is required by Scrum?

The Sprint Retrospective is one of the five events defined by Scrum. It is a formal opportunity for the Scrum Team to inspect itself and create a plan for improvements to be enacted during the next Sprint. The purpose of the Sprint Retrospective is to adapt the way of working so that it becomes more effective and enjoyable. The other options are not required by Scrum, but may be useful practices depending on the context. Developers do not have to stand up at the Daily Scrum, they can choose any format that works for them. A Sprint Burndown Chart is a common way to visualize the remaining work in a Sprint, but it is not mandated by Scrum. Release Planning is a topic that falls under the broader competency of Managing Products with Agility, but it is not a prescribed event in Scrum.

References:

The Scrum Guide

Understanding and Applying the Scrum Framework

Professional Scrum Product Owner™ I Certification