Final answer:
Antonia II informed Tiberius that Sejanus was plotting against him in 31 AD. It was not Germanicus, as he had died in 19 AD and was the adoptive son of Tiberius, while Sejanus was the prefect of the Praetorian Guard and later executed for his betrayal.
Step-by-step explanation:
In 31 AD, Antonia II (sister-in-law) informed Tiberius that Sejanus is plotting against him. While Germanicus, the adoptive son of Tiberius, had many achievements and intricate relations within the Roman power structure, it was Sejanus, the prefect of the Praetorian Guard and a trusted advisor of Tiberius, who was orchestrating a plot against the emperor.
Sejanus's ambition and ultimate betrayal became emblematic of the betrayals that Tiberius faced during his reign, leading to an atmosphere of paranoia towards the latter part of his rule. Germanicus died in 19 AD, before the time of this plot, so he could not have been involved in any conspiracy against Tiberius in 31 AD.
It is also well documented that the initial good relations between Tiberius and Sejanus turned sour as Tiberius grew increasingly wary of Sejanus's ambitions. The downfall of Sejanus in 31 AD came after Tiberius became convinced of his disloyalty and schemes.