Implicit memory is to explicit memory as is to )iconic memory; echoic memoryb)automatic processing; effortful processingc)short-term memory; long-term memoryd)the spacing effect; the self-reference effect

Implicit memory is to explicit memory as automatic processing is to effortful processing. So, B is the correct answer. You see, implicit memory is subconscious, in other words, it is automatic (you don't have to think about it, it just happens) whereas explicit memory happens when you are making an effort to remember something.