The correct option is B auxin to cytokinin
Callus is an unspecialized, unorganized, growing and dividing mass of cells. It is produced when explants are cultured on the appropriate solid medium, with both an auxin and a cytokinin in correct conditions.
Auxins and cytokinins, as growth regulators, have basic roles to play in plant tissue culture. Often, they are used together but with different concentration ratios, which subsequently determine the type of culture regenerated. A high cytokinin to auxin ratio supports the formation of shoots, whereas a high auxin to cytokinin ratio favours formation of roots. A balanced ratio favours production of callus.