At which place in humans is the skin the thickest?

The skin is thickest at (a) On the sole. The skin on the soles of the feet, specifically the plantar surface, is notably thicker compared to other areas of the body. This increased thickness is due to the presence of a thick layer of keratinized skin, known as the stratum corneum, which serves to protect the foot from friction, pressure, and abrasion associated with standing, walking, and other weight-bearing activities.
Similarly, the skin on the palms of the hands (b) is also relatively thick due to the constant use and exposure to friction and pressure. However, the skin on the soles tends to be slightly thicker than that on the palms.
In contrast, while the skin on the buttocks (c) and the head (d) may vary in thickness, they generally tend to be thinner compared to the skin on the soles and palms.