Avisual metaphor is effective because
a.
it provides a way to pass down stories from generation to generation.
b.
it is the only known artistic method that allows artists to convey both his/her tastes and ideas at the same time.
c.
it is a visual representation of a group or culture’s beliefs, which can be understood by all people.
d.
it allows an artist to use one or more images to replace another, in order to convey a message or idea.

Let's go through all the options:
a. it provides a way to pass down stories from generation to generation.
-there is nothing specific about generation transmitting in visual metaphors, so this is not the best option.

b. it is the only known artistic method that allows artists to convey both his/her tastes and ideas at the same time.
-it's not the only option! For example, literature allows to convey the two, and other means do too!

c. it is a visual representation of a group or culture’s beliefs, which can be understood by all people.
-yes, this is true, but a visual metaphor is also much more than that!

d.it allows an artist to use one or more images to replace another, in order to convey a message or idea.

- I think that this is the best answer (although I am not sure that "replaces another" means) - the visual metaphor's main function is to convey a message

The answer is gladiators