The best one would be "sole of the foot". As foot was not mentioned anywhere previously, just saying "sole" is wrong. "Bottom of the foot" is something I would avoid using as it implies that I don't know the name for the sole while "sole of the foot" is something that everybody knows. Good luck!

Suggestion in parenthesis ( ) The skin around eyelid is the thinnest (delete "skin", since you have already stated the skin around the eye, so redundant, (at) applox.(approximately, r not l ) 0.02 mm and the (sole of the foot) (has, not is) the thickest skin, (at) applox.(same as above, approx) 1.4 mm (I think you can delete the phrase "on the body" - not clear what you wanted to do with this phrase). In this context, sole means the "bottom of" and since you have not specified in this short paragraph where the sole is at the bottom of, specifying the foot is probably appropriate, although, IF you have specified it earlier elsewhere, then it may be possible to get away with just "the sole".... Hope this helps!