The correct answer is C. Tú eres.
In Spanish, "tú" (you) is used with the verb "ser" (to be) in the present tense as "tú eres." The options A ("yo estoy") and B ("ella está") use the verb "estar" (to be), which is different.
So, the complete sentence in a context emphasizing identity or intrinsic characteristics would be, "El teléfono es un gran uso del verbo 'ser.'"