Final answer:
Tiberius, adopted by Augustus after the death of potential heirs and the disgrace of his other grandson Agrippa, was married to Julia to secure an heir and eventually became the second emperor. The correct option is b.
Step-by-step explanation:
The nephew and son-in-law of Augustus, married to his daughter Julia to secure an heir, was Tiberius. Tiberius was adopted by Augustus after the untimely deaths of his prospective heirs, Gaius and Lucius, and the disgraceful behavior of his other grandson, Agrippa.
This adoption is a significant event in the history of the Julio-Claudian Dynasty as it led to Tiberius becoming the second emperor of the Roman Empire. The correct option is b.