What is the name of the alien species in 'The Forever War'?

The alien species in Joe Haldeman’s The Forever War is the Taurans.

The novel begins when humanity believes the Taurans have attacked and destroyed human colonist ships. Earth responds by creating the United Nations Exploratory Force, sending soldiers such as William Mandella to fight an interstellar war against them. The conflict lasts centuries by Earth’s calendar because near-light-speed travel and collapsar jumps create extreme time dilation.

The Taurans remain deliberately mysterious for much of the story. The war is eventually revealed to have begun through misunderstanding: humanity and the Taurans are both clone-based species, and their radically different biology and communication make peaceful contact difficult. Once meaningful communication becomes possible, the supposedly inevitable war is exposed as unnecessary.

The wrong options come from other science-fiction settings: Cylons belong to Battlestar Galactica, Buggers to Ender’s Game, and Formics are another name for Ender’s alien opponents. Taurans are specific to Haldeman’s novel.