A comma is used to indicate a pause, that would occur if the sentence were spoken aloud.
The given sentence emphasizes on the section of the mural painted by Siquerios, the 'centerpiece'. In order to create the required emphasis, the word 'centerpiece' must be followed by a comma. Thus, the correct answer is Option B.
Options C and D are incorrect - A semi colon (;) is used to join clauses of separate ideas in one sentence, which is not required in the sentence whereas, an em dash (---) is used to create a strong break to show an interruption, a change of thought.