For me it should be D because what if the user submit the form many times with the same email. The other data are maybe not necessary but if we can keep the email field as non-nullable and not unique with Primary Key

C. Create a data extension with email address as the Primary Key and reply date and response fields as nullable. Explanation: Since the email address is the primary identifier for subscribers, it should be set as the Primary Key to ensure uniqueness and efficient data retrieval. The reply date and response fields may not always be captured, so they should be set as nullable to accommodate cases where this information is not provided.