A sales rep owns an opportunity and can view the associated account, but is unable...

To allow account owners to selectively share contacts with opportunity owners, adding Opportunity owners to the Account Team and configuring Contact sharing is the recommended approach. Here’s why:

Account Team Access and Sharing: By adding the opportunity owner to the Account Team, the account owner can control access and selectively share related records, such as Contacts. Configuring Contact sharing settings allows the account owner to determine which contacts should be visible to opportunity owners.

Granular Sharing Controls: Using Account Teams with Contact sharing settings provides the necessary flexibility to control access at a more detailed level, ensuring that opportunity owners can see relevant contacts as needed.

Salesforce Best Practices: Salesforce recommends using Account Teams to manage record-level access for related records, providing targeted access to essential information.

In summary, adding Opportunity owners to the Account Team and configuring Contact sharing (Option B) is the best solution to allow selective sharing of contacts associated with accounts.