The excerpt best supports the conclusion that Mark Antony’s life will be spared because he will be useless without Caesar.
Brutus argues that once Caesar is dead, Mark Antony will not be able to do any harm and will be effectively powerless. He states that Antony is "but a limb of Caesar" and implies that without Caesar, Antony cannot pose a threat. Therefore, Brutus believes it is unnecessary to kill Antony as he will be diminished without Caesar’s influence.