The minimum stripping time of soffit formwork to beams refers to the amount of time that is required before the formwork supporting the concrete beams can be removed. This is an important consideration in construction as it ensures that the concrete has gained sufficient strength to support its own weight and any additional loads that may be applied to it.

The correct answer to this question is option 'C', which states that the minimum stripping time is 7 days. This means that the formwork can be removed after 7 days of the concrete being poured and allowed to cure.

There are several factors that determine the minimum stripping time of soffit formwork to beams:

**1. Strength of the concrete:**
Concrete gains strength over time as the cement hydrates and forms a solid structure. The minimum stripping time is determined based on the strength of the concrete at a particular age. In general, it is recommended to wait for the concrete to reach a compressive strength of at least 10 N/mm2 before removing the formwork.

**2. Type of cement used:**
Different types of cement have different curing times. For example, rapid hardening cement may achieve the required strength in a shorter period of time compared to ordinary Portland cement. The type of cement used in the construction will influence the minimum stripping time.

**3. Ambient conditions:**
The curing of concrete is also influenced by the ambient conditions such as temperature and humidity. Higher temperatures can accelerate the curing process, while lower temperatures can slow it down. The minimum stripping time may need to be adjusted based on the ambient conditions to ensure that the concrete has gained sufficient strength.

**4. Design requirements:**
The design requirements for the beams may also influence the minimum stripping time. If the beams are subjected to heavy loads or if they are part of a critical structural element, a longer stripping time may be required to ensure the stability and safety of the structure.

In conclusion, the minimum stripping time of soffit formwork to beams is determined based on the strength of the concrete, type of cement used, ambient conditions, and design requirements. Option 'C', which states a minimum stripping time of 7 days, is the correct answer based on these considerations.