Who invalidated or limited the effectiveness of laws passed to support the populists?
a) the u. s. supreme court
b) president howard taft
c) the u. s. senate
d) big business

It would be primarily "A) The U.S. Supreme Court" that invalidated or limited the effectiveness of laws passed to support the Populists, since during this time the court tended to side with "big business". 

Opposition to the nra came to a head in 1935, when the conservative supreme court declared the agency unconstitutional, claiming the nra gave the executive branch regulatory powers that belonged exclusively to congress. this decision began a series of court decisions that overturned key elements of the new deal.