The purpose of elevation in the post-operative care of a patient with a lower extremity amputation is to promote venous return and reduce swelling. Elevating the residual limb helps to decrease edema and improve blood flow, which can enhance healing and prevent complications such as deep vein thrombosis.
  
b. To improve the patient's comfort and relaxation: While elevation may improve the patient's comfort, the primary purpose is to address physiological concerns related to venous return and swelling.
c. To encourage early ambulation and mobility: Ambulation and mobility are essential components of post-operative care, but the specific purpose of elevation is not related to encouraging early ambulation.
d. To protect the surgical incision from contamination: While protecting the surgical incision is important, elevation is not the primary method for this purpose. Dressings and wound care techniques are used to protect the surgical site from contamination.