Final answer:
The standard curing time for concrete is typically 28 days, which allows for the required chemical reaction between cement and water to achieve expected structural strength and durability. So, the best answer is c, 28 days.
Step-by-step explanation:
The curing time for concrete is generally 28 days.
Concrete reaches the majority of its strength within the first few weeks of it being poured, and it continues to harden and strengthen over a long period of time.
However, for a practical timeframe, most construction standards consider 28 days as the official curing period at which concrete reaches its expected strength for structural use.
This time period allows for the hydration process, which is the chemical reaction between the cement and water, to occur sufficiently to provide the necessary durability and hardness.
So, the best answer is c, 28 days.