>>45
The far left one is the odd one out. The far right one is the only one of its size, the green is the only green, the circle is the only circle, the non-outlined square is the only non-outlined square. The far left one, the outlined square, isn't the "only" anything—there are other squares, there are other shapes of the same size, there are other outlined shapes, and there are other red shapes. It's the fact that it doesn't have something unique that makes it the odd one out.