This doesn’t appear in the pundit’s argument and therefore can’t be the main conclusion. We don’t know if new leaders will be more motivated to solve problems when national leaders are voted out.

This is very close to part of the support that the pundit uses to show why it’s good to vote national leaders out every few years. The author thinks that if leaders stay in office too long, they'll either deny that there is a problem or deny that they could have solved it. But this is offered to show why it's good to have leaders voted out of office after a few years. So (B) isn't the conclusion.

This is a premise that the pundit uses to show why voting out national leaders is a good thing.

This doesn’t appear in the pundit’s argument and thus can’t be a main conclusion. The pundit isn’t arguing about how much time governments should be given, but rather what voters should do every few years.

This accurately paraphrases the conclusion, which is the first sentence. The pundit concludes that it’s a good thing to vote out national leaders every year. Why? Because reforms are generally done early in a new government, not later. There's no problem with the word "should," because it's fair to interpret the claim that it's "good" to have leaders voted out after a few years as a recommendation that this should be done.