Plants maintain homeostasis by 1) opening and closing stomata. 2) converting excess sugar molecules to a lesser number of starch molecules 3) excreting waste through the cell membrane 4) all of the above

D)all of the above 

Homeostasis is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition within an organism. (Eg controlling amount of sugar in the bloodstream, keeping waste levels at acceptable levels.) 

Opening and closing stomata controls the gas levels within the plant. 
Converting excess sugar molecules to a lesser number of starch molecules regulates sugar levels. 
Excreting waste through the cell membrane prevents waste build-up.