The structure most responsible for maintaining cell homeostasis is the

Plasma membrane

Explanation:

The cell communicates with its outer environment with the help of cell membrane. The factors that keeps the cell healthy are involved in cellular homeostasis. The movement of ions and water is prevented through plasma membrane. Thus sodium ion concentration is maintained in extracellular environment. Inside the cell, organic acids and potassium ions concentration is high. The water movement across the membrane maintains the cytosol concentration. The cell tolerates temperature changes by maintaining fluidity of membrane. Thus, the structure most responsible for maintaining cell homeostasis is the plasma membrane.