Given that Robin’s soil feels very soft and similar to flour, and is not sticky, it is most likely D. silt.
Silt particles are fine and can feel floury or powdery when dry, yet they can also hold moisture without being sticky. Clay would typically feel sticky when wet, while sand has a gritty texture and would not feel soft like flour. Loam is a mixture of soil types and may not fit the description as directly as silt does.