Answer:
B. That the physical world is comprised of a single substance.
Substance monism is a philosophical view that asserts that there is only one fundamental substance that makes up the entire universe. In the case of substance monism about the physical world, it would be that all physical objects and phenomena can be reduced to and explained by this single fundamental substance. This is in contrast to substance dualism, which posits that there are two fundamental substances, such as mind and matter.
The statement "Idealism is a non-falsifiable theory" is a separate claim about the philosophical position of idealism, which holds that reality is ultimately mental or spiritual in nature, rather than material or physical. Whether or not idealism is falsifiable is a matter of philosophical debate, but it is not directly related to substance monism.