Decreasing grain size in a polycrystalline material has several effects on its mechanical and chemical properties. The correct answer is option 'A', which states that decreasing grain size increases yield strength and corrosion resistance. Let's explain this answer in detail:

1. Grain Size and Yield Strength:
- The yield strength of a material refers to the stress at which it begins to deform plastically. It is an important mechanical property that determines the material's ability to withstand loads without permanent deformation.
- Decreasing the grain size in a polycrystalline material increases its yield strength. This is due to the Hall-Petch relationship, which states that smaller grain sizes result in higher strength.
- In a polycrystalline material, grains act as barriers to dislocation movement. Smaller grains have more grain boundaries per unit volume, leading to increased resistance to dislocation motion and plastic deformation. This results in higher yield strength.

2. Grain Size and Corrosion Resistance:
- Corrosion resistance refers to a material's ability to resist degradation or deterioration due to chemical reactions with the environment.
- Decreasing the grain size in a polycrystalline material also improves its corrosion resistance. This is primarily because smaller grain sizes reduce the diffusion paths for corrosive species.
- Corrosion processes involve the transport of ions or molecules through the material. Smaller grain sizes restrict the movement of these corrosive species, making it more difficult for them to penetrate the material and initiate corrosion.
- Additionally, smaller grain sizes can lead to the formation of a more uniform and dense oxide or passivation layer on the material's surface, providing further protection against corrosion.

In summary, decreasing the grain size in a polycrystalline material has beneficial effects on its mechanical and chemical properties. It increases the yield strength by impeding dislocation motion and plastic deformation, while also enhancing corrosion resistance by reducing diffusion paths for corrosive species. Therefore, the correct answer is option 'A' - decreasing grain size increases yield strength and corrosion resistance.