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 ona 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.