Your company is facing a big problem of Duplicates. You have been asked to recommend an approach to ensure that the new leads are not already existing in the system, by evaluating their email. All leads must be blocked from entering the system. What would you recommend?

Answer options:

A.Create a validation rule to prevent duplicate leads from being entered
B.Create a Matching rule with Duplicate rules
C.Ensure all sales reps search Salesforce prior to entering data
D.Ensure that the email field is selected as “Unique”

Correct Answer: B 
A matching rule defines how duplicate records are identified in duplicate rules and duplicate jobs. Salesforce provides standard matching rules for business and person accounts, contacts, and leads. You can also create custom matching rules. For each matching rule, run on a record; up to 100 duplicates can be reassigned to a duplicate record set. A matching rule can be associated with a duplicate rule or a duplicate job. 
Option A is incorrect because with validation rules you can build more complex rules that have conditional statements to meet your business needs. 
Option C is incorrect because searching salesforce will waste the reps’ time and sometimes the record may or may not be displayed based on the search settings 
Option D is incorrect. Even if the email field is set to unique this will not prevent duplicates. 
Reference:

https://help.salesforce.com/articleView?id=matching_rule_map_of_reference.htm&type=5