Answer:
c. Colon
Explanation: The most common malignancy to metastasize to the gallbladder is colon cancer. The close anatomical proximity of the colon to the gallbladder and the common pathway of cancer spread through the lymphatic and hematogenous routes make the gallbladder susceptible to metastasis from colon cancer. Metastasis to the gallbladder from breast, lung, and melanoma cancers is less common.