The correct way to punctuate Aunt Martha's response to show that she was interrupted by the sound of the telephone is:
Looking at his grades, she spoke in an angry tone, “You have really disappointed me! You are—”
This indicates that her speech was cut off. However, since this option is not among the ones you've provided, the best alternative among your choices is:
Looking at his grades, she spoke in an angry tone, “You have really disappointed me! You are . . . ”
This indicates an incomplete thought due to interruption.