Which of the following sentences illustrates proper sentence structure?

Which of the following sentences illustrates proper sentence structure?

A. Everyone knows that a mango is much sweeter to an orange.
B. I saw the girl with my own eyes who said she had been abducted by aliens.
C. Either one of you should try and to fix their broken fence.
D. The queen had hardly finished her speech when the people began to disperse.

Answer - D