Final answer:
Greg should end the message by wishing Nadine the best in her future endeavors. Therefore, the correct option is D.
Step-by-step explanation:
When informing Nadine that she is no longer being considered for a promotion, Greg should end with wishing her the best in her future endeavors. While all other options may be helpful in certain situations, in this case, it is best to show support and respect for Nadine's future despite the disappointing news.