The ancient Athenians met together in a big meeting to discuss laws. Which of the following is the name for that big meeting?

You didn't give us a list of choices, but there's really just one choice. The Athenians big meeting for discussing laws was called the Assembly. Well, actually, in Greek (their language) it was called the "Ecclesia (ἐκκλησία). Etymologically, that means the "called out" ones. Or we might say the ones called together, to gather together. The Assembly was open to all male citizens, and could have several thousand people participating in such a meeting (out of the total citizen population of perhaps around 50,000).

By the way, the same term "ecclesia" was used by Christians later as the name for their gatherings, so in religious circles "ecclesia" (assembly) came to be synonymous with the word "church."