Electrons are emitted from a conductor when the conductor is
a. bombarded by high-speed electrons.
b. subjected to a small voltage.
c. cooled rapidly.
d. exposed to weak light.

Well, I have personally used both A and C (provided the light frequency takes it above the work function of the condutor). 

D could also work - a good conductor subject to a small voltage will carry a very large current. For instance, a flashlightg bulb filament is hot enough to emit electrons. 

B is a no go.