A researcher is documenting a formula and needs to write the expression: the lowercase Greek letter 'sigma' squared is equal to the sum, represented by the uppercase Greek letter 'Sigma', of 'x' subscript 'i' squared. Which of the following plain text representations correctly captures this mathematical expression?

Match each Greek letter symbol with its corresponding plain text representation used in mathematical notation.

The plain text commands \epsilon and \varepsilon both represent the lowercase Greek letter epsilon, but produce visually distinct symbols.