The forgotten leftovers had gotten quite old, he found it covered in mold in the back of his what?. Make the most suitable choice from thesecarpet, refrigerator, breadbox, fridge, coach
  The most suitable choice from the options provided would be "refrigerator". The forgotten leftovers had gotten quite old and were covered in mold in the back of his refrigerator.
 The most suitable choice for the sentence would be "refrigerator" or "fridge." Here is the completed sentence:

* The forgotten leftovers had gotten quite old, he found it covered in mold in the back of his refrigerator.

The other options (carpet, breadbox, and coach) are not suitable choices for this sentence because they are not places where food is typically stored and would not make sense in this context.