Premises
Most household appliances use electricity only when in use.
Many microwave ovens, however, have built-in clocks and so use some electricity even when they are not in use.
The clocks each consume about 45 kilowatt-hours per year.

Conclusion
Therefore, households without a built-in clock microwave use less electricity than households with a built-in clock microwave.

This is an assumption-based question. We need to look for an answer choice that when negated results in breaking the main conclusion that on average the households with no built-in clock microwave use less electricity than the households with no clock microwave.

A – The argument is not about microwave ovens in general. The discussion is about microwaves with clocks and without clocks. Option A is irrelevant to the argument. Eliminate.

B – The cost of a microwave is irrelevant. Eliminate

C – Having a gas oven or conventional oven is not relevant to the argument. Eliminate.

D – OA – Let’s negate option D. If households whose microwave does not have a clock use an extra clock then the power consumption for the households with both kinds of microwaves would be the same. This breaks the argument and hence is the right answer. Correct.

E – The argument is about the power consumed by types of microwave ovens. Eliminate.

Answer: D