The issue described occurs due to the settings in the master-detail relationship. Specifically:
The Allow reparenting checkbox, Child records can be reparented to other parent records after they are created, is unchecked (B). This setting prevents changing the parent record (contact) of a child record (service plan) once it has been set. If 'Allow reparenting' is unchecked, it restricts the ability to change the associated contact, leading to the issue observed.
The other options do not directly address the inability to change the associated contact on a service plan record. The 'Read Only' and 'Read/Write' settings (A and C) pertain to access rights to the detail records based on the master record's access but do not affect the ability to reparent a record.
For a detailed understanding of master-detail relationships and their settings, including reparenting, see the Salesforce documentation on relationships:
Master-Detail Relationships: https://help.salesforce.com/articleView?id=relationships_considerations.htm&type=5

To view a junction object record, a user must have at least read access to both master objects. This is because the junction object inherits the sharing and security settings from both master objects

To allow users access to Flows, Salesforce offers multiple integration points:
A . Quick Action: Flows can be initiated from record pages, the app utility bar, or global actions using Quick Actions. This allows users to execute flows relevant to specific records or global processes.
C . Flow Launcher: A component available in Lightning App Builder that enables users to start flows directly from a Lightning page.
Steps to set up these features:
For Quick Actions, navigate to Setup → Object Manager → select object → Buttons, Links, and Actions → New Action → Action Type (Flow).
For Flow Launcher, navigate to Lightning App Builder → select the page → drag the Flow component to the desired section of the page → configure it to launch the specific flow.
For more information on integrating Flows, check the Salesforce documentation on Flow Distribution.

Users will be unable to view the Account Interest records or the Interest records because they do not have read access to the Interest object. This is a standard behavior of Salesforce when a user does not have access to a related object.

The correct method to upgrade an unmanaged package that is installed in a production environment is to use the update link provided on the Installed Packages page. This process ensures that any modifications or additions in the package are properly integrated without the need to uninstall the previous version, thus preserving existing customizations and data.
A: Uninstalling removes all associated data and customizations, which is not advisable.
B: The Install Wizard is used for initial installations, not upgrades.