Bronislaw malinowski (1884–1942) advocated that ethnographers develop which skill during their fieldwork, a skill that brought the ethnographer clearly in communication within the local society he studied?

I believe the answer is: local language

The main job of ethnographers is to explore and understand the cultural phenomenon that happen in one society. In order to obtain the full information,  ethnographers need to directly communicate with the local people. Since we cannot expect them to learn our language, it is much better if we as ethnographers learn their language instead.

true because they can bargain

"choosing is refusing" means individuals face an opportunity cost with each decision they make. economists state that every decision involves trade-offs. opportunity cost is the least desirable alternative given up as a result of a decision.