Ans is Validation rules.
Even though Validation Rules only show one error per field, they do support showing multiple field-level errors simultaneously — which is what users usually want — with no JavaScript required.

So if:

You have validation rules targeting different fields (e.g., Email, Phone, Company),
And you're using lightning-record-edit-form with lightning-input-field,

Then when the user clicks Submit, Salesforce will automatically show all field errors inline — one per field — with no JavaScript needed.

I think it's A .
Salesforce validation rules are declarative and can be used to validate more than one field on an object and display multiple error messages. This approach requires minimal JavaScript code and uses Salesforce's built-in validation framework.

A is correct
https://developer.salesforce.com/docs/component-library/bundle/lightning-record-edit-form/documentation