Final answer:
Non-Greeks were the group expressly forbidden from attending the Eleusinian Mysteries, as they could not understand the Greek language, which was essential for the rituals and initiations of this ancient religious rite.
Step-by-step explanation:
The group that was expressly forbidden from attending the Eleusinian Mysteries was c. Non-Greeks. This ancient religious rite, significant in Greek history, was held in honor of the goddess Demeter and her daughter Persephone. It was based on secret initiations and rituals with a promise of rewards for this life and the life to come. The Eleusinian Mysteries included participants such as men, women, slaves, and free individuals, but one main condition was the ability to speak Greek.
This requirement was stipulated as attendees had to understand the proclamation of the festival. Furthermore, specific preliminary acts, including fasting and ritual acts, probably completed during the Lesser Mysteries, and the payment of a fee were part of the qualification to initiate, limiting the participation to those who could afford both the time and the fee. Given that the rite excluded those who could not speak proper Greek, we can discern that the inability to communicate in the Greek language was a barrier to participation.
This exclusion is also hinted at by the emphasis on Greek cultural norms, such as barefootedness with white clothing symbolizing purity, as well as the elaborate process of initiation that required knowledge of secret teachings possibly imparted by mystagogues who were already initiated.