Final answer:
The correct option is B. SQL
The skill required to effectively manage and manipulate event participation data and execute targeted follow-up emails is SQL, which is used for querying and operating on relational databases.
Step-by-step explanation:
The skill needed to build an efficient solution for the customer who wants to track event participation and follow up with customers is B. SQL. To cross-reference records in each registration data extension with the corresponding invitation email sends and to target customers who clicked on the registration link but did not complete the process, one would require the ability to query databases efficiently. SQL, or Structured Query Language, is the standard language used for managing and manipulating relational databases.
While AMPscript, HTML, and CSS are valuable skills in their respective areas for tasks such as customizing email templates or capturing form input, they would not be directly used to handle the data processing and segmentation tasks described. SQL is the most relevant skill for creating queries that identify the specific subset of users to be targeted for the follow-up email based on their actions related to the event invitations.