Why A? And why not the others?

Correct answer: A. Ojos del Salado

Ojos del Salado, a stratovolcano on the Chile-Argentina border in the Andes, rises to about 6,893 meters, making it the highest volcano anywhere on Earth. Mount Kilimanjaro is the tempting pick because it is Africa's tallest peak and one of the most famous volcanoes in the world, yet its summit reaches only about 5,895 meters, over a kilometer short. Cotopaxi is a genuine near-miss: it is regularly cited among the highest active volcanoes on the planet, but its roughly 5,897-meter summit still trails Ojos del Salado by nearly a kilometer. Mauna Loa is the real trap for anyone thinking in terms of sheer size rather than summit elevation — measured from its base on the Pacific seafloor it is the most massive volcano on Earth by volume, but as a broad, gently sloping shield volcano its summit sits at only about 4,169 meters above sea level, far lower than a steep-sided Andean peak.

Source: Encyclopaedia Britannica: 'Mount Ojos del Salado' (britannica.com/place/Mount-Ojos-del-Salado)