The correct option is:
B) Many workers were killed or sickened by malaria and yellow fever carried by mosquitoes.
When the United States gained control of the Panama Canal project (1904) the Isthmus of Panama was known for its tropical diseases. More than 22,000 deaths had occurred during the French effort to build a canal, especially due to yellow fever and malaria. It was later established that the poor living conditions of the workers included pools of standing water where mosquitoes were breeding and spreading disease.