When updating a record of a junction object in Salesforce (an object used to create a many-to-many relationship), you must have the appropriate permissions.
Required Permissions:
Read/Write access on the junction object: To update the junction object record itself.
Read/Write access on both master (parent) records: Because the junction object is in two master-detail relationships, and its security settings are controlled by the parent records.
Given the options:
Option A: Read/Write access on the primary relationship
Option B: Read/Write access on the secondary relationship
These options correspond to having Read/Write access on both parent objects.
Supporting Reference:
Master-Detail Relationships:
Permissions Inheritance: The detail (junction) record inherits sharing and security settings from the master (parent) records.
Updating Records: To update a detail record, users must have Read/Write access to both the detail record and its parent records.
Reference:
Why Option C is Not Sufficient Alone:
Option C: Read/Write access on the junction object
Insufficient Alone: While necessary, having Read/Write access on the junction object alone is not enough.
Must Combine with Parent Access: Users also need Read/Write access on both master records due to the master-detail relationship.
Conclusion:
To update a junction object record, users must have Read/Write access on both the primary and secondary relationships (Options A and B).
This ensures proper permissions in the context of master-detail relationships.

Scenario:
Universal Containers (UC) wants to allow multiple Accounts to share a default pickup address.
They have created a custom object called Global Address.
Requirement:
Create an efficient model to link multiple Accounts to a single Global Address.
Solution:
To associate multiple Accounts with a single Global Address, we need to create a lookup relationship from Account to Global Address.
Option D: Add a lookup field on the Account object to the Global Address object.
Correct Approach.
Adding a Lookup Field on the Account object that references the Global Address object allows multiple Accounts to reference the same Global Address.
This setup models a many-to-one relationship:
Many Accounts can have the same Global Address as their pickup address.
Benefits:
Data Normalization: Reduces data redundancy by storing the address once.
Ease of Maintenance: Updating the Global Address updates it for all associated Accounts.
An object cannot have two master-detail relationships to the same object.
Master-detail relationships imply ownership and cascade deletion, which may not be appropriate.
Does not address the requirement efficiently.
Option B: Add a master-detail field on the Global Address object to the Account object.
Incorrect Relationship Direction.
This would imply that each Global Address is owned by a single Account.
Does not allow multiple Accounts to share the same Global Address.
The master-detail relationship would be from Global Address to Account, which is opposite of what's needed.
Option C: Add a lookup field on the Global Address object to the Account object.
Incorrect Relationship Direction.
This creates a relationship where each Global Address references a single Account.
Does not support multiple Accounts sharing the same Global Address.
Would create a one-to-one relationship.
Conclusion:
Adding a Lookup Field on the Account object pointing to the Global Address object (Option D) creates a many-to-one relationship.
This efficiently models the business need where multiple Accounts can share a default pickup address.
Reference:
Lookup Relationships
Creating Relationships Among Objects
Why Other Options are Not Suitable:
Option A: Add 2 master-detail fields on the Account object to the Global Address object.
Incorrect.