Final answer:
The type of soil influences its field capacity, which is the amount of water soil can hold after it has been saturated and allowed to drain. Soil texture and organic matter content are critical in determining water retention. The correct option is e.
Step-by-step explanation:
The factor that will affect how much water a soil holds after a rain, also known as field capacity, is the type of soil. This is because the texture of the soil, which reflects the size of soil particles, significantly influences water retention capacities. Soils such as clay have many small pore spaces that can hold more water, in contrast to sandy soils which have larger but fewer pore spaces.
Additionally, organic matter and soil structure can influence water retention. Field capacity is reached when the soil holds all the water it can against the force of gravity, whereas at the permanent wilting point, plants can no longer extract the water they need from the soil.
Other factors that could potentially vary in an experimental setup, but are not directly related to field capacity, include individual plant variation, soil temperature due to different colors of containers, water loss due to evaporation from the soil, and the effect of insects.
Therefore, the correct option is e.