What Does It Mean When We Say a Vaccine Is

What does it mean when we say a vaccine is based on an "attenuated" virus?

A) The vaccine is a live virus that has been heat-killed.
B) The vaccine is a live virus that has been chemically damaged.
C) The vaccine is a similar species to the virus that isn't harmful (e.g., cowpox for smallpox) .
D) The vaccine isn't the virus at all, just similar-shaped chemicals.
E) The vaccine is based on a virus that has been rendered harmless by lab-induced mutation.