Final answer:
The question to identify the 1st commander and cousin of Claudius' first wife Plautia does not match any of the provided options correctly. Publius Ostorius Scapula is historically noted as the cousin but is not listed among the options, which include Cassius Chaerea, Sejanus, and others not directly linked to Plautia.
Step-by-step explanation:
The question asks about the 1st commander who was the cousin of Claudius' first wife Plautia. The correct answer is Publius Ostorius Scapula, often known simply as Ostorius. However, this name is not one of the provided options. The closest match from the options given is Cassius Chaerea, who indeed was involved in the assassination of Caligula, Claudius' predecessor. Nevertheless, he is not the cousin of Claudius' first wife Plautia. Often, in historical accounts, details can become muddled, and names such as Macro and Sejanus may appear in relation to Emperor Claudius. Sejanus was an ally of Claudius' predecessor Tiberius, and Macro was involved in the accession of Claudius. However, neither of these is known to be related to Plautia. Therefore, the provided options do not seem to match the historical facts as we know them.