Which One of the Following Distinguishes an Option as an American

Which one of the following distinguishes an option as an American style option?

A) option that grants its holder the right to purchase at the strike price
B) option that grants its holder the right to sell at the strike price
C) option that obligates its holder to sell at the strike price
D) option that can be exercised at any time prior to expiration
E) option that can only be exercised at expiration