OSCOLA REFERENCING TO BE USED please!

QUESTION TO BE ANSWERED- ‘To what extent have the principles of conferral, subsidiarity and proportionality helped to preserve the powers of the EU member states?’

Intended learning outcomes to be assessed are:
-demonstrate a clear understanding of the composition and function of the court of justice and the operation of the European unions legal system
-identify and critically evaluate the sources of European Union law including the legislative instruments and the legislative process
-demonstrate an understanding of the means by which EU citizens can gain redress through thier national courts on matters of EU law and how EU and member state law interact

Please include case law/relevant cases
2500 word limit