Final answer:
Among the options given, Second Corinthians is the pseudonymous book.
Step-by-step explanation:
The pseudonymous book among the options is Second Corinthians.
In the New Testament, Second Corinthians is considered pseudonymous because it is believed to have been written by someone other than the apostle Paul. Scholars have suggested that the author may have been one of Paul's followers who wrote in his name to convey their own theological ideas.
Unlike the other options listed, First Corinthians and First Thessalonians are generally accepted to be written by the apostle Paul himself, while Third Corinthians is not included in the canon of the New Testament.