What is the designation for the lowest social class within the plebeians?

The term "Plebs humilis" refers specifically to the lowest segment of the plebeian class within Roman society. The plebeians were the common citizens, distinct from the patricians, who were members of the aristocracy. Among the plebeians, "Plebs humilis" encompasses those who had the least social standing and were often associated with poverty or lower economic status. This designation highlights their place within the broader hierarchical structure of Roman society.

In comparison, "Augustales" referred to a particular group of priests dedicated to the worship of the emperors and the broader imperial cult, which does not pertain to the social class structure. "Ingenui" refers to freeborn citizens, not specifically the lowest class within the plebeians, while "Manumission" relates to the act of freeing a slave, rather than a class designation within the plebeians. Hence, "Plebs humilis" fits as the most precise answer to the question regarding the lowest social class within the plebeians.