How is a soil characterized if most of its particles are of similar size?

When a soil is characterized by having most of its particles being of similar size, this condition is referred to as uniformly graded. In uniformly graded soils, the particle size distribution is narrow, meaning the sizes of the grains do not vary significantly. This homogeneity can affect various properties of the soil, such as its density, permeability, and compaction characteristics.

In contrast, a well-graded soil has a wide range of particle sizes that include small, medium, and large particles, contributing to good compaction and stability. A gap-graded soil has a specific range of sizes where some sizes are missing, leading to voids that can affect drainage and strength. Coarse-graded soils predominantly consist of larger particles and may not exhibit the characteristics of uniformity throughout their particle size distribution.

Thus, the definition of uniformly graded aligns precisely with the description given in the question, making this the correct choice.