Please clarify your question! "Major organ of d" is not a clear phrase. What are you referring to with "d"?

For example, are you asking about:

* The digestive system? In that case, the major organ is the stomach.

* The male reproductive system? Then the major organ is the testis.

* A specific organ? If so, please provide more context.

Once I understand what "d" refers to, I can help you with the answer.