The mechanism that cells use to move these small potassium molecules into the cell against the concentration gradient active transport by endocytosis (option B).
Active transport is the movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.
Active transport is used to move molecules and substances against a concentration gradient i.e. from regions where they are low to regions where they are high.
According to this question, active transport by endocytosis (process by which the plasma membrane of a cell folds inwards to ingest material) can be used to move pottasium ions.