Final answer:
The missing element of closing a project is confirming the results with the company's customer, which is vital for ensuring that the improvements meet the customer's needs and are acknowledged by them.
Step-by-step explanation:
The element of closing a project that is missing is Confirming the results with the company's customer (Option C). When closing out a project, it is essential to communicate results to the stakeholders, including the customers who were the focus of the DMAIC project's improvements. Furthermore, producing a final project report for management review (Option D) not only provides documentation of the project's success but also offers insights for future projects. Celebrating with the team and reallocating resources are also important, but the primary missing element is ensuring that the improved process meets the customer's needs and that they acknowledge the results.