The correct answer is C. 3NF (Third Normal Form). According to CompTIA DataSys+, Third Normal Form is widely regarded as the most preferable and practical level of normalization for relational database design in real-world environments. It strikes an optimal balance between reducing data redundancy, maintaining data integrity, and preserving query performance.

To understand why 3NF is preferred, it is important to consider the progression of normalization. First Normal Form (1NF) ensures that tables contain atomic values and no repeating groups. Second Normal Form (2NF) builds on this by eliminating partial dependencies, ensuring that non-key attributes depend on the entire primary key. While these forms improve structure, they do not fully address all redundancy issues.

Third Normal Form (3NF) goes further by removing transitive dependencies, meaning that non-key attributes depend only on the primary key and not on other non-key attributes. This significantly reduces duplication of data and prevents common anomalies during INSERT, UPDATE, and DELETE operations. CompTIA DataSys+ emphasizes that eliminating transitive dependencies is critical for maintaining long-term data consistency and integrity.

Although Fourth Normal Form (4NF) and higher normal forms exist, DataSys+ notes that they are typically applied only in specialized cases involving complex multi-valued dependencies. Over-normalization beyond 3NF can increase schema complexity, require excessive joins, and negatively impact performance without providing proportional benefits in most transactional systems.

As a result, 3NF is considered the industry-standard target for relational database design. It provides a clean, maintainable schema that supports scalability, reduces redundancy, and aligns well with performance expectations. CompTIA DataSys+ highlights 3NF as the most commonly implemented and recommended normalization level for operational databases.

Therefore, the most preferable normal form for relational database design is 3NF, making option C the correct and fully verified answer.