Final answer:
Of the options, First Corinthians is definitely not pseudonymous and is widely accepted as authentically written by the Apostle Paul. The correct option is 2.
Step-by-step explanation:
The student is asking about the authorship of certain books within the Christian New Testament. Specifically, they're inquiring about which book is definitely not pseudonymous, meaning not written under a false name.
Of the four options provided, First Corinthians is widely accepted by scholars as being authentically written by the Apostle Paul without question, which makes it definitely not pseudonymous.
The other three books, Second Thessalonians, Titus, and First Timothy, have been debated among scholars with some considering them pseudonymous. However, First Corinthians retains a strong scholarly consensus on its authenticity and Pauline authorship. The correct option is 2.