The correct answer is "isn't eating".
Here's why:
* The sentence is in the present continuous tense (is at the table), so the missing verb should also be in the present continuous tense. This tense describes an action happening now.
* "Isn't eating" is the correct negative form of the present continuous tense for the subject "she".
Therefore, the complete sentence is: Lisa is at the table, but she isn't eating anything.