Which of the following nutrient elements is responsible for the die back of citrus?

Answer: Option B

Cu is the nutrient element that is responsible for the dieback of citrus.
Citrus dieback, also known as 'chlorosis' or neglectosis is not a specific disease but a syndrome expressing many disorders in the plant that leads to a decline in productivity, reduced productive life, and poor fruit quality.
Copper-deficient plants show dark green and enlarged leaves. Sterns are more slender than normal and appear flat and angular. Limbs die back and gumming may occur under new bark or on fruits.