Answer: I cut my finger with a stick that I found in the pond.
Explanation: This sentence is best because every other sentence is grammatically incorrect. In the first sentence you would be adding an extra sentence, when you can make it one. In the third sentence you put a period in the middle of a perfect sentence, only to make another one. In the fourth sentence you are implying that the pond cut you rather then then stick. And lastly in the fifth sentence, the fifth sentence just isn't proper English.
