Concept:
Modern mural art became very powerful in Mexico during the twentieth century.
Mexican artists used murals to express social, political, cultural and historical ideas.
The Mexican mural movement had a major influence on modern public art.
Step 1: Understand modern mural movement.
Modern mural art is not only decorative.
It is also used to communicate public messages and national identity.
Step 2: Identify Mexican muralists.
Artists such as Diego Rivera, David Alfaro Siqueiros and Jose Clemente Orozco created a revolution in mural art.
They painted large public murals connected with society, workers, revolution and national history.
Step 3: Final conclusion.
Since the modern mural movement became revolutionary in Mexico, the correct answer is:
\[
\boxed{\text{Mexican}}
\]
Hence:
\[
\boxed{\text{(C)}}
\]