Texture:
Soil texture refers to the relative proportions of sand, silt, and clay particles in the soil. These particles determine the texture of the soil, which in turn affects its properties and behavior. The texture of the soil affects various aspects such as water retention, drainage, aeration, and nutrient availability.

Water Holding Capacity:
The water holding capacity of the soil refers to its ability to retain water for use by plants. This capacity is influenced by the size and arrangement of soil particles. Soil with a higher proportion of clay particles has a greater water holding capacity compared to sandy soil. This is because clay particles have smaller spaces between them, which allows them to hold onto water more effectively.

Field Capacity:
Field capacity is the amount of water held by the soil after excess water has drained away and the soil is no longer saturated. The field capacity of the soil is influenced by its texture. Soils with a high clay content have a higher field capacity because clay particles have small spaces that can hold more water.

Soil Flora:
Soil flora refers to the community of microorganisms, including bacteria, fungi, and other organisms, that live in the soil. While soil particles do not directly determine soil flora, they do indirectly influence it. The texture of the soil affects properties such as water retention and nutrient availability, which in turn impact the growth and activity of soil flora. Different soil textures can support different types and quantities of soil flora.

Conclusion:
In conclusion, soil particles determine the texture of the soil, which has a direct influence on its properties such as water holding capacity and field capacity. These properties, in turn, affect the growth of soil flora. Therefore, the correct answer to the question is option 'A' - texture.