C option right because rate of hydration of given substance in hydration process live followers this manner to that c2s bouge compound like hydration rate is fast then other substances so if put the all other substances in increasing rate of hydration then we get like this type of series

Cement is a key ingredient in the production of concrete, and it is primarily composed of four main compounds: tri-calcium silicate (C3S), di-calcium silicate (C2S), tri-calcium aluminate (C3A), and tetra-calcium alumino-ferrite (C4AF). These compounds contribute to the hydration process of cement, which is the chemical reaction that occurs when water is added to cement and leads to the formation of solid concrete.

The rate of hydration refers to the speed at which the cement compounds react with water to form hydration products. Different compounds have different rates of hydration, and their contributions to the overall hydration process can vary. In this question, we are asked to identify the compounds in decreasing order of their rate of hydration.

Below is the explanation of the correct answer, option 'C':

C34 C3S, and C2S

1. C3S (Tri-calcium Silicate):
- C3S is the most abundant compound in cement, typically comprising about 50-70% by weight.
- It reacts rapidly with water and contributes significantly to the early strength development of concrete.
- The hydration of C3S produces calcium silicate hydrates (C-S-H) gel, which is responsible for the strength and rigidity of concrete.

2. C34 (Tetra-calcium alumino-ferrite):
- C34 is the compound that reacts the fastest among all the cement compounds.
- However, it has a relatively small contribution to the overall hydration process as it is present in smaller quantities in cement (typically around 4-12% by weight).
- The hydration of C34 produces calcium aluminate hydrates (C-A-H) gel, which contributes to the early setting and hardening of concrete.

3. C2S (Di-calcium Silicate):
- C2S is the second most abundant compound in cement, typically comprising about 15-30% by weight.
- It has a slower rate of hydration compared to C3S but still contributes significantly to the overall strength development of concrete.
- The hydration of C2S also produces calcium silicate hydrates (C-S-H) gel, which contributes to the strength and durability of concrete.

In conclusion, the correct order of the compounds in terms of their rate of hydration is C34, C3S, and C2S. However, it is important to note that all four cement compounds play vital roles in the hydration process and contribute to the overall properties of concrete.