Let's break down the stimulus. This is an argument and the conclusion is, "Photovoltaic power plants offer a less expensive approach to meeting demand for electricity than do traditional power plants."
How do we know this? Well, the cost of producing power at photovoltaic power plants is one-tenth of what it was 20 years ago, whereas the corresponding cost for traditional plants has increased.
Notice that the conclusion is comparing the cost at photovoltaic plants with the cost at traditional power plants but the premises do not compare these plants. We only know that the cost for photovoltaic is 1/10 what it was 20 years ago (i.e. comparing photovoltaic to photovoltaic) while the cost at traditional power plants is increasing (i.e. comparing traditional with traditional). This information, however, would not allow us to conclude that photovoltaic is less expensive.
Since this is a Strengthen with Sufficient Premise question, we are looking for the premise that guarantees the conclusion of the argument. We need an answer choice that relates photovoltaic plants being 1/10 as expensive as it was 20 years ago with the cost of traditional plants today. The only answer choice that relates the cost of photovoltaic plants 20 years ago with the cost of traditional plants is answer choice (D).
(D) states that 20 years ago, the cost of producing electric power at photovoltaic plants was less than 10 times the cost of producing power at traditional plants. If the cost of photovoltaic is 1/10 what it was 20 years ago and 20 years ago the cost of photovoltaic was less than 10 times the cost of traditional plants, this would 100% guarantee the conclusion of the argument.
Some simple numerical examples will clarify. Assume that 20 years ago, the cost of photovoltaic plants was $20. Today it would be $2 (i.e. 1/10 what it was 20 years ago). (D) is now telling us that $20 is less than 10 times the cost of producing power at traditional plants. So let's assume that the cost at traditional plants 20 years was $3 (i.e. 10 x $3 = $30 > $20). Today, the cost of photovoltaic (i.e. $2) is less than the cost of traditional (i.e. more than $3 because the stimulus states the cost of traditional plants has increased).
Hope this helps! Please let us know if you have any other questions.
