Complete the sentence with the most appropriate word or phrase out of the given options.
She is sitting ____ a big stone.

on

The most appropriate word to complete the sentence is on.

The other options are not as accurate. Over suggests that she is sitting above the stone, but this is not the case. At suggests that she is sitting near the stone, but this is not the most accurate description. Above suggests that she is sitting higher than the stone, but this is not the case either. On is the best option because it suggests that she is sitting directly on the stone.