In this clip, we see four-year-old bradley performing a conservation task. bradley has difficulty completing the tasks because he is at what piagetian stage of cognitive development

The correct answer is the preoperational stage of cognitive development. 

During ages 2-7, children typically fall under the preoperational stage of cognitive development. During this stage of development, children are able to adopt symbolic thinking, by using words and drawings to express things or ideas. However, during this stage of development, children lack the ability of conservation. This means that children do not understand  that the quantity of something remains the same despite changes to its appearance, layout or orientation.