The correct answer is option 'A': 1 and 2.

Concrete is a widely used construction material due to its strength, durability, and ability to be molded into various shapes. However, it is important to understand the factors that can affect its behavior, including shrinkage. Shrinkage refers to the reduction in volume or dimensions of concrete over time.

1. Relative Humidity of the Atmosphere:
The relative humidity of the atmosphere plays a significant role in the shrinkage of concrete. When the surrounding air is dry, it can absorb moisture from the concrete, causing it to shrink. Conversely, when the air is humid, the concrete can absorb moisture from the atmosphere, leading to expansion. Therefore, the relative humidity of the atmosphere affects the moisture content in the concrete and consequently its shrinkage.

2. Passage of Time:
As concrete cures and ages, it undergoes a process called hydration, where the cementitious materials react with water to form a hardened mass. During this process, some of the water is chemically consumed, while the rest evaporates. The loss of water through evaporation causes the concrete to shrink. This shrinkage continues over time as the concrete dries and reaches its equilibrium moisture content. The longer the time since the concrete was placed, the more significant the shrinkage will be.

3. Applied Stress:
While applied stress can cause deformation in concrete, it does not directly influence its shrinkage. Shrinkage is primarily a result of moisture loss and changes in relative humidity, rather than the applied stress. However, it is worth noting that stress can affect the cracking tendency of concrete. When concrete shrinks, it exerts tensile forces, which can lead to cracking if the applied stress exceeds its tensile strength.

In summary, the correct statements regarding the shrinkage of concrete are:

1. Shrinkage of concrete depends upon the relative humidity of the atmosphere.
2. Shrinkage of concrete occurs over time as the concrete dries and reaches its equilibrium moisture content.

Therefore, the correct answer is option 'A': 1 and 2.