Explanation
The correct answer is B because SQL can be used to cross-reference the records in each registration data extension with corresponding invitation email sends, and to send a follow-up email to customers who click on the registration link but do not complete registration. A is incorrect because AMPscript cannot be used to cross-reference records or send follow-up emails. C is incorrect because HTML cannot be used to cross-reference records or send follow-up emails. D is incorrect because CSS cannot be used to cross-reference records or send follow-up emails.