Which one doesn't belong?

Here are four flower pictures I took photos of when on a walk. Which one doesn't belong?

This is an example of a WODB activity. The concept was popularised by Christopher Danielson, author of the book with the same title. Four things (shapes, objects, numbers ...etc) are displayed and students are asked 'Which one doesn't belong?' In a well designed set, each of the four should be a possible contender for different reasons. More background information can be found here.

I saw the symmetry within the flowers but there are so many other things to look at too. I shared a couple of sets of pictures on twitter and tagged them #WODB. Here they are being used in classrooms!

Images from twitter:  

I love this activity as a starting point to encourage discussion and reasoning as well as vocabulary and understanding.