In following questions, one term in number series is incorrect. - Find out the incorrect number 1, 3, 10, 21, 64, 129, 256, 778

Answer: D. 256

The sequnece is * 2 + 1, * 3 + 1, * 2 + 1, * 3 + 1,....
So, 256 is wrong and must be replaced by (129 * 2 + 1)  i.e.  259.