Hi,
Basically, relearning is the most sensisitive because it allows for a greater detection of information stored in memory. For example, if you were asked to recall the definition of learning ten years in the future, you are unlikely to be able to recall it. The one retrieval cue is ineffective in allowing the activation and retrieval of relevant node/concept.
Recognition is better. Why? Because you have a lot more retrieval cues, hence more likely to activate relevant node (think about spreading activation)
But, relearning is the best. Because you are going over the information, using relearning as a tool helps to demonstrate that hey, this information is still stored. How does it do this? Well, it shows that because you take a shorter period of time or a smaller number of trials to 'relearn' the same information, information (the amount varies) must be still stored in memory. Therefore, it has the greatest ability to detect the storage of information when compared to the other two.
Hope this helps!