The three benefits of using a partner development org over an individual development org in this scenario are:

Partner development org has a greater number of licenses available for the team. This means that the developers can test their app with different user profiles and permissions, and also invite beta testers to provide feedback2.

Partner development org offers higher API limit. This means that the developers can use more API calls to integrate their app with external systems or services, and also leverage the Salesforce APIs to automate their development and deployment processes2.

Partner development org offers more storage. This means that the developers can store more data and files in their org, and also use more features and components that require storage, such as Big Objects, Platform Events, or Lightning Web Components2.

The other options are not benefits of using a partner development org over an individual development org in this scenario, because:

Only partner development org supports managed beta testing. This is not true, as both partner development org and individual development org can create and distribute managed beta packages3.

Partner development org never expires. This is also not true, as partner development orgs have a 12-month expiration period, which can be extended by request