Final answer:
Derived traits in primates include forward-facing eyes and opposable toes, with the former potentially being an adaptation for predation. Mammary glands and having one offspring at a time are not uniquely derived in primates but are common to all mammals.
Step-by-step explanation:
Among the given traits, forward-facing eyes and opposable toes are examples of derived traits in primates. Forward-facing eyes are believed to be adaptations for hunting insects and other small prey in the forest undergrowth, which is a trait not only shared by primates but also by predators that prey on small animals. This suggests that they may have evolved in primates as an adaptation for predation rather than an arboreal lifestyle. The opposable toes are part of the specialized adaptations for climbing and grasping, which are characteristic of primates but not necessarily ancestral to all mammals.
On the other hand, mammary glands are not a derived trait in primates as all mammals possess them. Similarly, having one offspring at a time is also not an exclusively derived trait for primates as it is common among various mammalian groups, including eutherians, which all primates belong to.