To solve this problem, we need to identify the component in the given figure that has thin outer walls and highly thickened inner walls.$\newline$Let's analyze the image:$\newline \bullet$The figure shows a cross-section of a stoma with labeled parts A, B, C, and D.$\newline \bullet$In a typical stoma, the guard cells are the components that control the opening and closing of the stomatal pore.$\newline \bullet$Guard cells have a unique structure with thin outer walls and highly thickened inner walls.$\newline$Now, let's match this description with the labeled parts:$\newline \bullet$Part C is the guard cell, which matches the description of having thin outer walls and thickened inner walls.$\newline$Therefore, the correct answer is Option 3: C.