We are told that the only fingerprints found on Mr. Tannisch's premises were his own. From this the argument concludes that Mr. Tannisch's guest's missing diamonds must have been taken by someone who wore gloves.
What's the flaw here? Well, we haven't yet ruled out an "inside job." The only fingerprints found were Mr. Tannisch's. So, why have we ruled out Mr. Tannisch being the thief?
Likewise, answer choice (A) describes a situation where the option of "inside job" is being ignored. All of the campers who became ill this afternoon ate only food from the camp cafeteria. We have not yet ruled out the possibility that the food at the cafeteria was contaminated. So, it is flawed to conclude that the cause of the illness must not have been something they ate.
As you can see, answer choice (A) and the argument in the passage have the same flaw in their reasoning.
Hope that clears things up! Please let us know if you have any other questions.
