Final answer:
The correct answer to the question is Diamond Head (a), a volcanic tuff cone near Waikiki used as a military lookout in the 20th century.
Step-by-step explanation:
The popular hiking destination just beyond Waikiki, which was used as a military lookout in the 20th century, is Diamond Head (a). Diamond Head is a volcanic tuff cone on the Hawaiian island of O'ahu and is a well-known landmark close to Waikiki Beach.
The other options, Mount St. Helens (b), is located in Washington state and is famous for its major eruption in 1980. Mauna Loa (c) and Haleakala (d) are both volcanoes in Hawaii. Although Mauna Loa is one of the most active volcanoes on Earth, located on the Island of Hawaii, and Haleakala is a massive shield volcano that forms more than 75% of the Hawaiian Island of Maui, neither is the correct answer to the question.