Most likely, people who perform tough manual work can develop

The correct option is: thick epidermis on their palms

Step-by-step Explanation:

1. Understanding the Structure of the Skin:
The human skin is composed of multiple layers, the outermost of which is called the epidermis. The epidermis acts as a protective barrier shielding the body from environmental hazards, pathogens, and physical trauma.

2. Response to Mechanical Friction and Pressure:
When individuals engage in tough manual work, their hands and palms are subjected to continuous and repeated mechanical friction, rubbing, and pressure from handling rough tools or heavy materials.

3. Hyperkeratosis and Epidermal Thickening:
In response to this repetitive physical stress, the skin activates a natural protective physiological mechanism. The rate of cell division in the basal layer of the epidermis increases, leading to an accumulation of keratinocytes (skin cells) and keratin in the outermost layer (stratum corneum). This localized thickening of the epidermis is called hyperkeratosis, commonly known as a callus.

4. Why Other Options are Incorrect:
- Greater quantity of melanin all over the body: Melanin production is triggered by exposure to ultraviolet (UV) radiation from sunlight, not by mechanical manual labor.
- Greater number of sweat pores in their hands: The number of sweat glands and pores is genetically determined during development and does not increase in response to physical work.

Thus, people who perform tough manual work are most likely to develop a thick epidermis on their palms to protect the underlying tissues from damage.