Answer:
B. Remove the comma after the word know and add that
Step-by-step explanation:
In the given sentence, a comma after the word know should be removed and the word that should be added.
The phrase Little did (someone) know means that the person in question was completely unaware of something. In the given example, the explorers weren't aware of the fact they found one of the oldest art displays in the world. This phrase should be followed by a conjunction such as that, which is why option B is the correct one,