Weathering of rocks:
- Weathering is the process by which rocks are broken down into smaller pieces or undergo chemical changes due to exposure to various environmental factors such as wind, water, and temperature changes.
- As rocks undergo weathering, the minerals present in them are released and become available for other processes to act upon.
- This weathering process can be physical, such as the freezing and thawing of water in cracks, or chemical, such as the reaction of rock minerals with water or acids.
- The weathering of rocks plays a significant role in the enrichment of minerals in rocks because it breaks down the rock into smaller particles, exposing more surface area for chemical reactions to occur.

Erosional and depositional action of rivers:
- Rivers play a crucial role in the transportation and deposition of sediments.
- As rivers flow, they erode the land, carrying sediments such as rocks, minerals, and soil particles.
- These sediments are transported downstream and eventually deposited when the river's velocity decreases, such as in river bends or at the mouth of the river.
- During this process, minerals that were weathered from rocks in the upstream areas are carried by the river and deposited in new locations.
- The erosional and depositional action of rivers helps in redistributing and concentrating minerals, thereby contributing to the enrichment of minerals in rocks.

Both (a) and (b):
- Both weathering of rocks and the erosional and depositional action of rivers contribute to the enrichment of minerals in rocks.
- Weathering breaks down rocks, releasing minerals and making them available for other processes.
- Rivers then transport these minerals and deposit them in new locations, contributing to the enrichment of minerals in rocks.
- The combined action of weathering and rivers helps in the concentration and redistribution of minerals, ultimately leading to the enrichment of minerals in rocks.

Therefore, the correct answer is option 'C' - Both (a) and (b).