The correct option is A Zinc
Auxins are phytohormones that play a major role in various physiological activities of plants. Zinc is essential for the synthesis of tryptophan, which is a precursor of auxins like IAA (Indole-3-acetic acid). Hence, zinc is needed for the biosynthesis of auxins.
Molybdenum is a component of several enzymes like nitrogenase that helps in nitrogen fixation.
Copper is involved in several redox reactions while potassium helps in protein synthesis, cation-anion balance and opening and closing of stomata.