The best answer is B. The need for the team to learn based on doing work and measuring results.
The Sprint length should be chosen based on the complexity and volatility of the product and the environment, as well as the team's capability and Definition of Done. The Sprint length should enable the team to deliver a usable Increment of value that meets the feedback needs of the stakeholders and the business. The Sprint length should also allow the team to inspect and adapt their work processes and practices based on the outcomes of the Sprint.
The other options are not valid reasons for choosing the Sprint length. The organization's mandate, team formation, and release schedule should not dictate the Sprint length, but rather be aligned with it. The Scrum Team should have the autonomy to choose the Sprint length that best suits their product and context.
You can learn more about how to choose the right Sprint length from these sources: How Long A Sprint Should Be?, How to Choose the Right Sprint Length in Scrum, What is a Sprint?, Determining the Sprint Length, and Sprint Length: What's the Right Length?.

According to the Scrum Guide, the timebox for a Sprint Planning event is 8 hours for a one-month Sprint, proportionally shorter for shorter Sprints. The other options are not valid, as they are either too long (such as monthly), too vague (such as whenever it's done), or too short (such as 4 hours for a one-month Sprint).