Step 1: In cryoglobulinemia, abnormal antibodies in the blood clump together when the temperature drops, then dissolve again on warming.
Step 2: The infection most famously behind mixed cryoglobulinemia is chronic Hepatitis C, which keeps the immune system producing these cold-precipitating proteins.
Step 3: The remaining options - an ovarian malignancy, diabetes and leukemia - are not recognised classic triggers, leaving Hepatitis C as the answer.
\[\boxed{\text{Hepatitis C}}\]