Answer: The correct answer for the blank is- A) Have hairs.
As per the University of Michigan, there are primarily three characteristics/ features that are unique to mammals. These are as follows-
1) Presence of hairs on body.
2) Presence of mammary glands ( primarily used of feeding the neonates).
3) Presence of three bones in the middle ear ( that is Malleus, incus, and stapes).
The most significant function of hairs in mammals is that they provide insulation against cold by conserving heat of body. In certain nocturnal animals ( which are active during night time), they serve as sensory organs.
Thus, option A) is the right answer.