The correct answer is option 'c' - C3A, C3S, and C2S.

Explanation:
The rate of hydration refers to the speed at which cement compounds react with water to form a solid structure. Cement is primarily composed of four main compounds: C3A (tricalcium aluminate), C3S (tricalcium silicate), C2S (dicalcium silicate), and C4AF (tetracalcium aluminoferrite).

1. C3A (Tricalcium Aluminate):
C3A is the compound that contributes the most to the rate of hydration. It reacts rapidly with water to form calcium aluminate hydrates. This rapid reaction results in the initial setting and early strength development of cement. However, excessive amounts of C3A can lead to the formation of undesirable compounds such as ettringite, which can cause delayed expansion and cracking in concrete.

2. C3S (Tricalcium Silicate):
C3S is the next compound in terms of contributing to the rate of hydration. It reacts with water to form calcium silicate hydrates (C-S-H), which are responsible for the long-term strength and durability of concrete. The hydration of C3S is relatively slower compared to C3A but still significant in the overall hydration process.

3. C2S (Dicalcium Silicate):
C2S is the least reactive compound among the three mentioned. It hydrates more slowly than C3A and C3S, but it continues to contribute to the strength development over a longer period. The hydration of C2S results in the formation of additional C-S-H gel, which further enhances the strength and durability of concrete.

In summary, the rate of hydration of cement compounds decreases in the following order: C3A, C3S, and C2S. However, it is important to note that all four compounds contribute to the overall hydration process and properties of cement. The proportions of these compounds in cement can vary depending on the specific type and composition of cement being used.