Among the following, which one is not an ape?

Gibbons, gorillas and orangutans are members of the hominoid (ape) clade: gibbons are the ‘lesser apes’ (family Hylobatidae) while gorillas and orangutans are classified among the great apes—thus options 1, 2 and 4 are apes [1]. Langurs, in contrast, belong to the Old World monkey family (Cercopithecidae, subfamily Colobinae) and are classified as monkeys rather than apes [2]. Taxonomically, apes (Hominoidea) differ from monkeys by features such as absence of a tail and distinct skeletal and digestive adaptations; colobine langurs retain a tail and other monkey-specific traits. Therefore the langur is not an ape but an Old World monkey [2].

Sources

1. [1] https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/hominoids
2. [2] https://www.emory.edu/LIVING_LINKS/pdfs/primate_taxonomy