The grid in a triode is kept negatively charged to prevent ...

-the variations in voltage from getting too large

-the electrons from being attracted to the grid instead of the anode

-electrons from escaping from the tube containing the triode

I think the correct answer is the third statement, electron from escaping from the tube containing the triode. The negative charge on the grid repels any electron in the tube. As a result, the flow of current is controlled. If the field is that strong, all current flow will stop resulting to maintaining the electron cloud in the tube.