Which executive agency would most likely investigate cases of espionage?
a the central intelligence agency
b the department of state
c the department of defense
d the securities and exchange commission

The first alternative is correct (A).

The executive agency dealing with espionage cases is the Central Intelligence Agency (CIA).

The CIA is the body responsible for conducting secret investigations in search of suspicious activity that could jeopardize national security. In this sense, the CIA provides strategic information to the government, which takes appropriate measures to protect the American nation and, eventually, the world.