Which species is primarily semi-aquatic and lives in family groups?

The muskrat is primarily semi-aquatic and lives in family groups, making it the correct choice for this question. Muskrats are social creatures that often inhabit wetlands, marshes, and ponds, creating intricate burrows and lodges along the water's edge. They tend to live in family units, which can include a breeding pair and their offspring, working together to forage for food and maintain their habitats.

Other species listed may exhibit some semi-aquatic behaviors, but do not primarily live in family groups like muskrats. Mink are typically more solitary and do not form family groups in the same way. River otters are also semi-aquatic and highly social, often found in groups known as rafts, but the question specifically seeks a species known for living in family units rather than just social groups. Raccoons can be found near water, but they are not classified as semi-aquatic and do not typically form family groups; they are more solitary or form loose aggregations. Thus, the muskrat stands out as the species that fits the characteristics described in the question.