255 is correct Answer. Please read the question properly. look at the fist and third conditions.
1) You must give oranges, 1/2of how much u have and also 1/2 of one orange
3) Don't cuting the oranges
If 128 then how will u satisfy above condition with out cutting the orange. And also look at the second condition.
2)- after giving the portion of last (seventh guard) guard u have balance only one orange.