Final answer:
The correct phrase to complete the sentence is 'Je préfère'.
Step-by-step explanation:
The correct phrase to complete the sentence is Je préfère. The sentence will then read as 'Je préfère la tablette parce qu'elle est très chère. Je préfère le lecteur MP3.' This translates to 'I prefer the tablet because it is very expensive. I prefer the MP3 player.'
Learn more about Completing a sentence in French