Which one of the following is monogamous?

Wolves (Canis lupus) are typically socially monogamous: wild canids commonly form long-term pair bonds and breed as a single male–female pair, a pattern documented across the family and emphasized for gray wolves in comparisons with related taxa (social monogamy is typical of gray wolves) [2]. By contrast, pinnipeds such as seals and walruses exhibit mating systems characterized by male competition and polygyny or other non-monogamous tactics typical of many marine mammals, with pinniped mating tactics and polygynous tendencies described in the literature on harbor seals and related species [4]. Ungulates like deer are also generally polygynous (rutting males mate with multiple females). Thus the monogamous choice is wolf.

Sources

1. [1] https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2019.00341/full
2. [2] https://www.pnas.org/doi/10.1073/pnas.2421756122
3. [3] https://www.researchgate.net/publication/226692255_Mating_tactics_and_mating_system_of_an_aquatic-mating_pinniped_The_harbor_seal_Phoca_vitulina
4. [4] https://www.sciencedirect.com/topics/physics-and-astronomy/reproductive-strategy