Final answer:
The Sanitary Commission's significance was in promoting public health measures during the Civil War, particularly by improving camp sanitation to reduce disease mortality among soldiers and assisting with medical care. Option A is correct.
Step-by-step explanation:
The significance of the Sanitary Commission formed during the Civil War in California was A. Promoting public health measures. The United States Sanitary Commission, established in June 1861, played a crucial role during the Civil War by inspecting military camps to improve cleanliness and sanitation to reduce the number of soldiers who died from diseases, which constituted the most common cause of death during the conflict. They were also instrumental in raising funds for medical supplies and assisted with caring for the injured. Women, in particular, volunteered in significant numbers for the Commission, in addition to taking on roles such as cooks and laundresses for the Union army.