The correct answer is option 2: Magnesium. Yellow spots on citrus leaves can indicate a deficiency of Magnesium. It is essential for the proper functioning of chlorophyll, the pigment responsible for photosynthesis. When there is a lack of Magnesium, chlorophyll production is affected, resulting in yellow spots on the leaves.
Option 1: Zinc is not the correct answer in this case. Zinc deficiency can cause a different set of symptoms, such as stunted growth and distorted leaves.
Option 3: Boron deficiency may also cause yellowing of citrus leaves, but it is more commonly associated with brown or necrotic patches rather than spots.
Option 4: Iron deficiency is characterized by yellow leaves with green veins, which is different from the yellow spots described in the question.
Hence, the correct answer is option 2: Magnesium.