To answer this question the first step we have to follow is to assign the oxidation numbers to each of the elements in the reaction (remember that pure elements have an oxidation number of 0):
From this, we can observe that the elements that changed their oxidation numbers were K and Cu.
K changed its oxidation number from 0 to +1, which means that it lost an electron, it was oxidized and it is the reducing agent.
Cu changed its oxidation number from +2 to 0, which means that it gained 2 electrons, it was reduced and it is the oxidizing agent.
The reducing agent in this reaction is K.
The correct answer is the fourth choice.