The correct answers are A. Change the Lead Duplicate Rule details to Bypass Sharing Rules and D. Change the Lead Duplicate actions to Block on Create.

Because Lead OWD is Private, users may not have access to existing duplicate Lead records. Salesforce duplicate rules can be configured to bypass sharing rules, allowing the duplicate rule to check possible duplicates regardless of ownership or user visibility. Salesforce Help specifically explains duplicate prevention can block users from creating duplicate records whether or not they have access to the existing record.

D is also required because detecting a duplicate is not enough. The duplicate rule action must be set to Block on Create to stop users from saving new duplicate Leads.

Why others are incorrect:

B is incorrect because Lead Assignment Rules assign leads; they do not prevent duplicate creation.

C may expose more Lead records, but changing OWD from Private to Public Read is not the best security-safe solution when duplicate rules can bypass sharing.

===========