The development of barrier potential in the depletion zone of a PN junction is primarily due to the diffusion of majority carriers across the junction. Here's a detailed explanation of why option 'A' is the correct answer:

### Understanding the PN junction
A PN junction is formed when a P-type semiconductor (with an excess of holes as majority carriers) is brought into contact with an N-type semiconductor (with an excess of electrons as majority carriers). This contact creates a depletion zone at the junction, which is a region devoid of majority carriers.

### Diffusion of Majority Carriers
1. Initially, when the P and N regions are brought together, the free electrons in the N-region near the junction start diffusing into the P-region.
2. Similarly, the holes in the P-region near the junction start diffusing into the N-region.
3. This diffusion of majority carriers occurs due to the concentration gradient of carriers on either side of the junction.
4. As a result, the N-region near the junction loses some of its free electrons, creating a region with a net positive charge. Similarly, the P-region near the junction loses some of its holes, creating a region with a net negative charge.
5. This movement of majority carriers across the junction creates a space charge region or depletion zone.

### Formation of Barrier Potential
1. As the majority carriers diffuse across the junction, they leave behind positively charged donor ions in the N-region and negatively charged acceptor ions in the P-region.
2. The diffusion process continues until the buildup of the electric field due to the ionized atoms in the depletion zone opposes further diffusion.
3. This electric field creates a potential difference across the depletion zone, known as the barrier potential.
4. The barrier potential prevents any further diffusion of majority carriers across the junction, establishing equilibrium.
5. The magnitude of the barrier potential depends on the doping concentrations of the P and N regions and can be controlled during the fabrication of the PN junction.

### Conclusion
In summary, the development of barrier potential in the depletion zone of a PN junction is a result of the diffusion of majority carriers (electrons and holes) across the junction. This diffusion process creates a space charge region and establishes a potential difference, which is known as the barrier potential.