Option 1 -> unscrupulous means lacking moral principles, which relates to ethics rather than consistency of behavior.
Option 2 -> tenacious means holding firmly to something; actually a synonym of persistent, not an antonym.
Option 3 -> vacillating means wavering or being indecisive; the opposite of being persistent or constant.
Option 4 -> determined means firmly decided; another synonym of persistent, not an antonym.
Hence, Option 3: vacillating -> A persistent salesman continues firmly without wavering, while a vacillating person wavers and is indecisive—these are opposite behaviors -> correct