Explanation
Auto response rules are a way to automatically send email responses to leads or cases based on certain criteria such as lead source, industry, etc. They can be used to send personalized emails whenever a lead fills out a web-to-lead form on a website and send different messages based on the lead industry field value. Using validation rule to trigger workflow to email the lead is not possible because validation rules cannot trigger workflows or send emails; they only prevent records from being saved if they do not meet certain criteria.
Adding a public group and process builder to email the lead is unnecessary because auto response rules can handle this requirement without additional configuration or customization. Creating an assignment rule to email the lead is also unnecessary because assignment rules are used to assign leads or cases to users or queues based on certain criteria, not send emails; although they can have email alerts as part of their actions, they are not as flexible as auto response rules for personalizing email messages. References:
https://help.salesforce.com/s/articleView?id=sf.customize_leadsautoresponse.htm&type=5