The prime lending rate is a key rate in the economy: not only are the interest rates on most loans to small and medium-sized businesses tied to the prime, but also on a growing number of consumer loans, including home equity loans.
This sentence uses idiomatic paired coordinators, not only...,but also.., to relate two basic kinds of loans to the prime lending rate: (1) loans to small and medium-sized businesses and (2) consumer loans. A, the best choice, is the only one that maintains the necessary parallelism in the phrases following the paired coordinates: not only on..., but also on.... Choices B and D omit the on after but also. Choice C (not only the interest rates ..., but also on) is not parallel either. Choice C especially garbles the meaning.