Discarding and replacing worn appliances may seem to be a wasteful thing to do, but in many cases, a
new appliance is actually cheaper than fixing the old one.
A. a new appliance is actually cheaper than fixing the old one
B. a new appliance is actually cheaper than an old one is
C. buying a new appliance is actually cheaper than fixing an old one
D. it is actually cheaper to buy a new appliance than fixing an old one.
E. a new appliance is actually cheaper to buy than it is to fix an old one
In a comparison we have to compare equal forms. In this case we could either compare two nouns "purchase" (it doesn't appear here, but just theoretically) and "fix" or two verb forms. It is important that the verb forms are parallel to eachother.
The only answer that obeys this rule is C) in which we compare "buying" with "fixing", which are parallel forms. I hope my explanation makes sense.