What argument was Oliver Wendell Holmes making in this quotation about how our federal system works?

“I do not think the United States would come to an end if we [the Court] lost our power to declare an Act of Congress void. I do think the Union would be imperiled if we could not make that declaration as to the laws of the several States.” —Justice Oliver Wendell Holmes
a
Laws passed by State governments are so varied and numerous that the Federal Government should have no say over their constitutionality.
b
All State laws should be reviewed by the Supreme Court to keep the federal system running.
c
The Supreme Court needs to be able to settle disputes between the States and the Federal Government by reviewing the constitutionality of State laws.
d
Congress does not pass unconstitutional laws, so there is less need for the Supreme Court to review those laws.