23ts

[3UMLy60M]

asdfhe45rt

[MiiNiPaa]

You already have input validation in one place:

		cin >> choice;

		// Validate the menu selection
		while (choice < 1 || choice > 5)
		{
			cout << "Please select a valid choice of 1-5: ";
			cin >> choice;
		}

Just make something similar in each case.

[CroCo]

you have to validate your numbers for each input.

if (choice == 1)
		{
			// Get the length of the rectangle
			cout << "Please enter the length of a rectangle: ";
			cin >> rectangleLength;
			while (!(rectangleLength > 0))
			{
				cout << "Please select a positive number: ";
				cin >> rectangleLength;
			}

			// Get the width of the rectangle
			cout << "Please enter the width of a rectangle: ";
			cin >> rectangleWidth;
			while (!(rectangleWidth > 0))
			{
				cout << "Please select a positive number: ";
				cin >> rectangleWidth;
			}

			// Display the area of the rectangle
			area(rectangleLength, rectangleWidth);
		} 

Go through all you choices