The correct answer is (D) Open highland, usually treeless. Also, any of certain grass-covered ranges of chalk hills in South England, particularly the North Downs (Surrey and Kent) and the South Downs (Sussex).

Downs comes from Old English dún (hill).

Examples of use:

• She parked her auto and, carrying a butterfly net, descended the downs.
• He hoped to enter one of his horses one day in a race at Epsom Downs.