The retention of five digits on the hands and feet of primates is a variable trait in the order characteristic only of monkeys characteristic only of apes and humans the only truly distinctive primate trait none of these'

The answer is ‘the only truly distinctive primate trait’. Prehensile hands are a distinctive trait of primates an opposable thumb (capable of grasping). The digits have  nails instead of claws, and tactile pads enriched with sensory nerve fibers at the ends of digits to feel to manipulate objects.