Stait followed by Magellan at the southernmost reaches of his voyage?
Check this map -- near the southern tip of South America for the name of his strait.
http://panama-maps.com/images/mapscomoptimized-south-america-map.gif
Straight of Magellan
To determine which strait Magellan followed at the southernmost reaches of his voyage, we need to know that Magellan embarked on the first circumnavigation of the Earth from Spain in 1519. During his voyage, he reached the southernmost tip of South America, now known as Cape Horn. To continue his journey to the Pacific Ocean, Magellan needed to find a passage through the southern tip of South America.
The strait that Magellan followed is called the Strait of Magellan, named after him. It is located between the mainland of South America and the large island of Tierra del Fuego. Magellan and his crew navigated through this treacherous waterway between late 1520 and early 1521.
So, to answer your question, Magellan followed the Strait of Magellan at the southernmost reaches of his voyage.