Q:

According to Ravi Zacharias's apologetic argument, how could love have existed prior to the fall?

Ravi Zacharias explained in a Q&A (14:50) that for true love to exist between God and man, the current creation is the only possible way for it to exist. He gave four possible examples of creation, but knowing good from evil results in perfect freely chosen love between man and God.  
According to Zacharias, how is it possible then for a loving relationship between man and God to have existed in the Garden of Eden? (man did not know good from evil)  Only when the serpent mislead man, good and evil was revealed. How could Adam and Eve have lived in a state of true love with God?  

A:

You appear to be slightly misunderstanding Zacharias's argument.  Here are his four options:

No world
Amoral world (no such thing as good and evil)
Constrained world (no possibility of choosing evil)
Free world (possibility of choosing evil)

Out of these options, he says that the fourth option, the actual Creation, is the only one in which love is possible.
Perhaps the root of your confusion is what is meant by the "knowledge of good and evil."  The Genesis account makes it clear that Adam and Eve had the option of disobeying from the very beginning; otherwise there would have been no need for God to give them a commandment (Genesis 2:17).  And of course the Fall is the ultimate demonstration of man's freedom to choose evil.
This awareness of the possibility of disobedience is different from the "knowledge of good and evil" associated with the forbidden fruit.  Zacharias explains that knowledge in these terms:

Eating the fruit from the tree of the knowledge of good and evil basically gave humanity the power to redefine everything. God had given language, identification, and reality to humankind. He imparted to humans the power to name the animals. But essential to the created order was a moral framework that the creation was not to name or define. This was the prerogative of the Creator, not of the creation. I believe that this is what is at stake here. (source)

So the correct way to understand Zacharias's argument here is that Creation has always been "option #4," both before and after the Fall.

