Final answer:
The classical diagnostic sign of scabies is the presence of burrows on the skin, created by the mite Sarcoptes scabiei.
Step-by-step explanation:
The classical diagnostic sign of scabies is A) Burrow. Scabies is a skin infestation caused by the mite Sarcoptes scabiei, which creates tunnels or burrows into the upper layer of the skin where it resides and lays eggs. These burrows appear as tiny, irregular, silvery-white or skin-colored lines on the skin surface which can be intensely itchy, especially at night. Other skin eruptions such as vesicles, pustules, or petechiae can occur as well, but the burrow is the pathognomonic sign that can confirm scabies.