This is correct and very succinct! For anyone who is not quite as versed in critical reasoning here is a little more on the topic of cause and effect.

When you find that a critical reasoning question concludes that one thing is the cause of another then you have a cause and effect relationship. In this case the supposed cause is the presence of sunlight and the observed effect is increased sales.

The effect is always true and cannot be debated. In this question it is true that the departments under the skylight do in fact have higher sales. What is is doubt is the reason why the sales are higher. You can imagine many other reasons besides the sunlight. It could be that the items in that section are more popular or more expensive or that these are necessities that everyone needs to purchase. Perhaps the section under the skylight has recently been renovated and so it is a more pleasant place to be regardless of the sunlight. So there are lots of other potential causes.

If you wanted to weaken the argument you could simply mention one of these other causes. This would cast some doubt. This is called offering an "alternate cause." This is a frequent way to weaken cause and effect.

To weaken you could also show the same effect occurring without the cause. For example, the same departments could have higher sales in other Savefast stores even though these other stores do not have skylights. This would show that the effect occurs without the cause and would weaken. Similarly you could show that the cause occurs without the effect. So if 10 other stores with skylights showed no differences in sales that would also weaken.

Of course in this case we are looking to strengthen. And to strengthen we can do the opposite of the things that would weaken. So instead of mentioning an alternate cause we could have an answer choice that blocked an alternate cause. It could say something like - the departments under the skylight are not more popular than the other departments. Thereby blocking one alternate cause.

We could also show that the cause leads to the effect in other situations. Such as showing that in other department stores with skylights the departments receiving natural light have higher sales.

And finally we can do what is done in this case. As manishcmu mentions this answer choice B is a case of "no cause therefore no effect." What this means is when the cause is taken away - obviously there is no sunlight coming through the skylights at night - the effect goes away as well - sales are not higher in those departments at night.

In summary, for cause and effect questions:

To weaken:

1) Show Alt Cause
2) Show the Cause without the effect
3) Show the effect without the cause


To Strengthen:

1) Block an alternate cause
2) Show that the cause leads to the effect in other situation(s)
3) Show that without the cause the effect goes away as well.