Buenas is a greeting in spanish that is often used in place for all of the options Good Morning, good afternoon and good evening, that expression "Buenas" would be like a howdy or something of the sort, just a greeting and a farewell for whoever you are saying hi at the street.
B) esta. esta typically refers to describing something like location whereas es describes the person's attributes. (ie, esta would be used with "she is in the store" and es would be used with "she is cute"
