Thank you for your question. I always love a good science competition. First off, let me refer you to a Science Buddies project: "Veggie Power! Making Batteries from Fruits and Vegetables" at this link:http://www.sciencebuddies.org/science-f ... background
It provides a lot of good information for an experiment of this sort. Rather than just provide the answer of which fruit/veggie will provide the most voltage, I have a feeling that the point of the exercise assigned is to conduct some research and experiment to optimize/maximize voltage output. With that, lets discuss some of the processes involved here.
In this application, electricity is produced from a chemical reaction between two dissimilar metal electrodes immersed in an electrolyte solution. The electrolyte, in this case, is provided by the fruit/vegetable cell. You can narrow down good fruit/veggie candidates by researching ones that are high in electrolytes, such as Potassium. You can then test the candidates to find which ones are the most efficient at producing electricity.
Also, it's not just the fruit/veggie cell that contributes to the voltage output. You must also consider the type and placement of your electrodes. A) They must be two different metals, i.e. an anode and a cathode, (I'll leave the research as to which two types work best up to you) or the reaction will not work. B) Electrode orientation and distance from one other will play a factor in how efficient the circuit is (i.e. parallel, end to end, touching or not, etc...).
Some additional questions to get you thinking: Does the size of the fruit/veggie make a difference? What happens if you cut the fruit/veggie up into smaller individual cells (you said you could use up to 10 individual fruits/veggies, but you didn't say you couldn't cut them up and have 20 or more cells)? Does the circuit produce more voltage if the fruit/veggie cells are connected in series or in parallel? Does cell temperature make a difference in voltage output? Does freshness of the cell make a difference? How long before you deplete the power potential of an individual cell? I'm sure there are other questions, but these should get you started.
Please post back with additional questions or comments. I look forward to hearing how you did in the competition.
I hope this helps.
"As the circle of light increases, so does the circumference of darkness around it."
~ Albert Einstein