Differential to single ended
1.Transformer


If your signal is a single, or narrow band, frequency this method is fine. However, anything else and you introduce the transformers frequency response into your signal.


  3.simply ground the negative part with matching a load


This method is ok, IF the signal source is close to the receiver. Since you are are throwing away the common mode noise reduction that is the reason the signal was sent differential in the first, this method should be limited to part to part connection locally on a PCB.


  2.Amplifier


A differential amplifier is ultimately your best choice. Not only will it be easier to control the frequency response of the receiver, but you also take full advantage of the common mode noise removal. Further, this method decouples the input signal from wherever the signal is headed to giving you the appropriate output impedance to feed the next stage.