Final answer:
The longest skin cell is estimated to be 1 millimeter in length, smaller than 10 and 100 millimeters, but definitely larger than a single micrometer.
Step-by-step explanation:
The estimated length of the longest skin cell is 1 millimeter (mm). Considering that a typical human red blood cell is about 8 micrometers (µm) in diameter and that 250 of them could fit on the head of a pin, which is about 2 mm in diameter, skin cells are larger but not as large as 10 millimeters or 100 millimeters, nor as small as a single millimeter. Therefore, the correct answer is a) 1 millimeter.