Answer:
The sentence that uses the word "whereas"incorrectly is B. No one knew the whereas of the little dog, who suddenly disappeared.
Explanation:
The word "whereas" is used to compare or contrast with a truth or fact. It is correctly used in option A and C where it is used to compare/contrast.
The correct word to be used in option B would be whereabouts since they did not know where the dog is. Using whereas when looking for the dog does not show any comparison or contrast.
Whereabouts is used to show where something is.