Which type of scar is known to be harmless and secondary to trauma?

The type of scar known to be harmless and secondary to trauma is a hypertrophic scar. These scars form as a natural part of the healing process after an injury and are characterized by raised and thickened skin. Unlike keloids, which are also raised but extend beyond the original site of injury, hypertrophic scars remain within the boundaries of the wound and may improve over time.

The other options do not fit the description of a harmless scar secondary to trauma. Keloids are excessive growths of scar tissue that can be uncomfortable and may require treatment. Actinic keratosis refers to a condition resulting from sun exposure that can lead to skin lesions, not scars. Melanoma is a form of skin cancer that can arise from moles or other skin lesions, which is certainly not harmless and requires medical attention.