What is on a transplanted organ that causes the transplant recipient's body to reject it?

Major histocompatibility complex (MHC) antigens

Explanation:

The process in which recipient body rejects the transplanted organ is called a allograft rejection or transplantation rejection.

A transplanted organ contains Major histocompatibility complex (MHC) antigens which are required to be accepted by the recipient's body to function properly but T cells in recipient's body become alloreactive and recognize MHC antigens (donor-derived antigens) as foreign and starts making antibodies against it. Which means the recipient's body is rejecting the  transplanted organ.

Hence, the correct answer is "Major histocompatibility complex (MHC) antigens".