Final answer:
The word 'shibboleth' refers to a custom, word, or phrase used as a test to distinguish members of one group from others; it historically involved pronunciation but now broadly signifies any identifying trait of group membership.
Step-by-step explanation:
The word 'shibboleth' is originally a Hebrew word that has been adopted into English. It historically refers to a word or phrase that was used to distinguish members of one group from outsiders. In modern use, a shibboleth can refer to any custom, principle, or belief distinguishing one group of people from others.
In that sense, it's not any of the provided options but closest to option 2, a way of pronouncing a word, since the original Biblical use involved pronunciation as a test of nationality. In the Book of Judges in the Old Testament, Gileadites used the word 'shibboleth' to identify and slay Ephraimites, who could not pronounce the 'sh' sound.
This term has evolved to represent any kind of linguistic or cultural test used to signify whether an individual belongs to a certain group or not. While the word's earliest uses were more literal, categorizing it into specific contemporary contexts often leans towards describing an identifier of social affiliation or group membership.