The main barrier that prevents the entry of micro-organisms into our body is

The correct option is Skin.

The human immune system relies on multiple layers of defense to protect the body against pathogens (disease-causing microorganisms). The first line of defense consists of physical and chemical barriers that work to prevent these invaders from entering the body in the first place.
Here is a step-by-step breakdown of why the skin serves as the primary barrier:

1. Physical Shield (First Line of Defense):
The skin is the largest organ of the human body, covering its entire outer surface. The outermost layer of the skin, called the epidermis, is made of tightly packed, dead cells containing a tough, waterproof protein called keratin. This structure forms an unbroken, highly impermeable wall that physically blocks bacteria, viruses, and fungi from penetrating into deeper tissues.

2. Chemical Defense:
Beyond acting as a physical wall, the skin produces sweat and sebum (skin oil). These secretions contain lactic acid and fatty acids that create an acidic surface environment, typically in the range of:
$$\text{pH}=3to5$$
This acidic environment is hostile to most microorganisms, inhibiting their growth and survival on the skin.

3. Why the other options are incorrect:
• Antibodies: These are Y-shaped proteins produced by B-lymphocytes. They act within the body's fluids to target and neutralize pathogens, but only after those pathogens have already crossed the external barriers and entered the body.
• Macrophages: These are specialized white blood cells that engulf and digest cellular debris and foreign microbes. They form part of the internal, non-specific cellular defense (second line of defense) and only encounter pathogens that have already gained entry.
• Monocytes: These are white blood cells circulating in the blood that migrate into tissues and differentiate into macrophages. Like macrophages, they function internally and do not prevent initial entry.

In summary, while the other options fight infections internally, the skin is the main physical barrier that blocks microorganisms from entering the body.