Thanks for your question.
The short answer is that we don't know what the fruit was.
However, it doesn't matter what it was. Could it have been an apple? Sure. A peach? Why not? A kumquat (is that a fruit?)? Could be. But we aren't told so we don't know. The point is that as soon as God designated the fruit of that tree, whatever it was, to be off limits it became the tree of the knowledge of good and evil. It became the focal point of their decision to obey God or to disobey Him. It was at that tree that the first decisive battle of faith was fought and lost.