The correct answer is (B).
(B) Weaken
Step 1: Identify the Question Type
A question stem that asks you to identify evidence against an argument is a Weaken question. This stem asks you to weaken the scientist's explanation. The classic way to weaken an argument with an explanatory conclusion is to provide an alternative explanation.
Step 2: Untangle the Stimulus
As is often the case for a Strengthen or Weaken question, the question stem directs you to the conclusion, i.e., the "explanation of Earth's warming." The first sentence simply states that the Earth has warmed by 0.5 degrees Celsius. The second sentence contains the scientist's explanation: The warming is caused by the accumulation of gases in the atmosphere, which blocks the planet from releasing heat.
Step 3: Make a Prediction
This is a classic LSAT argument pattern in which an author points out an interesting phenomenon and then provides a causal explanation for that phenomenon. The built-in assumption is that no other explanation exists. The typical way to weaken such an argument is to look for any of the classic alternatives to causation: The causation is reversed, a third factor causes both, or the correlation is merely coincidence. You don't need to predict the specific cause; just keep in mind that you need an answer that indicates one of those three situations is at play.
Step 4: Evaluate the Answer Choices
(B) is correct. A great way to weaken a claim of causality is to show the result preceded the purported cause. If the bulk of the warming occurred before the greatest buildup of the gases, then it is less likely that the buildup caused the warming.
(A) is Out of Scope. The argument is concerned only with the cause of the global warming, not the cause of the gas buildup. Additionally, it never mentions industrial pollution, so the amount of gasses that arose from pollution (whether it's "some" or "many" or "all") is irrelevant.
(C) is appealing as it might initially seem to provide an alternative explanation for the warming: increased solar radiation. However, all this choice really indicates is that solar radiation was not constant year over year. To provide an alternative explanation for the average increase in Earth's temperature over the past century, there would need to be a trend of an average increase in solar radiation over the past century, not just yearly fluctuations.
(D) is irrelevant because it includes no reference to time or change. There is no indication that the amount of volcanic dust increased or decreased over the past century, so this choice can't explain the changing temperature.
(E) is a 180 because it strengthens the argument. If the buildup of minor gases was uncharacteristically greater in the past century, then it is more likely that the built up gases may be responsible for the trapping of heat and, therefore, also responsible for higher temperatures.
