Protists are living beings that are eukaryote (they have a nucleus and membrane-bound organelles). Protists can move by the movements of cilia and/or flagella, but those are active movements (they demand energy and an active phisiology of the organism). The same goes for amoeboid movement, as an amoeba needs to change the shape of its own body in order to move. So, the only way in which protists can move passively, without making any effort, is through air or water currents, as most protists are microorganisms and are susceptible to air and water currents.
Therefore, the correct answer is B. through air or water currents.