Ask Mark
Mark seems to be the man in charge of the fruit tree growing in his backyard, so before you pick some, you need to ask him for permission. Seems fair enough, how many times have you caught neighbors stealing fruit from your trees? Technically it really isn’t their property and they should ask first.
They don’t know what you were planning to do with those pieces of fruit. Perhaps you wanted to make a pie or harvest them yourself for whatever reason. Or maybe you didn’t want to do anything with them, just watch them all go to waste, you have the right to that as well, after all, it is your fruit.