Option 1: Thick epidermis - The epidermis is the outermost layer of the skin. While it plays a role in protecting the skin, it is not directly responsible for developing thick skin on the palms and soles.

Option 2: Thick dermis - The dermis is the layer beneath the epidermis. It contains collagen fibers that provide strength and resilience to the skin. Developing thick skin could be attributed to the thickening of the dermis to withstand constant pressure and friction.

Option 3: Thick subcutaneous tissue - The subcutaneous tissue is the layer of fat beneath the skin. While it provides insulation and padding, it is not directly responsible for developing thick skin on the palms and soles.

Option 4: All of these - The correct answer indicates that all the mentioned factors (thick epidermis, thick dermis, thick subcutaneous tissue) contribute to the development of thick skin on the palms and soles in labourers who do hard manual labor.