Explanation
A Lookup Filter is the recommended method to meet the requirement of preventing service agents from selecting entitlements that are not associated with the account assigned on the case. A Lookup Filter is a type of filter that restricts the values and records that are available in a lookup field based on criteria and conditions. A Lookup Filter can be used to limit the entitlements that are available in the Entitlement Name lookup field on the case page layout based on the Account Name field value. This way, service agents can only select entitlements that belong to the same account as the case. Verified References: Service Cloud Consultant Certification Guide & Tips, Define Lookup Filters