Which of the following practices is not allowed in Scrum?

The practice that is not allowed in Scrum is Hardening Sprints. In Scrum, a Sprint is meant to create a potentially shippable product increment at the end of each iteration. The idea behind Hardening Sprints is to set aside a specific time after a series of Sprints to address technical debt, test, or polish the product before release.

However, this contradicts the core principles of Scrum, which promotes a sustainable pace of work and continuous delivery. By intending to allocate a separate Sprint for hardening, it implies that the team has not been addressing quality and technical debt continuously throughout the previous Sprints, which goes against the concept of delivering potentially releasable increments after every Sprint. Scrum encourages teams to integrate development, testing, and any necessary adjustments during the planned Sprints to ensure that each increment is of high quality without extending the timeline artificially with hardening.

Daily Standups, Product Backlog refinement, and Sprint Retrospectives are all fundamental Scrum practices that promote transparency, continuous improvement, and effective communication within the team. These practices are fully aligned with the Scrum framework's goal of enhancing collaboration and ensuring that the team is delivering value effectively.