Sort each statement by whether it is true of $\mathbf{E}$ or of $\mathbf{D}$.

True of $\mathbf{E}$: Exerts the force on a test charge, Is reduced inside a dielectric at fixed charge

True of $\mathbf{D}$: Responds to deliberately placed charge alone, Is unchanged by inserting a dielectric at fixed free charge

$\mathbf{E}$ responds to every charge, bound included, so it falls inside a dielectric, and it is what pushes a test charge. $\mathbf{D}$ tracks only free charge, so at fixed free charge it is untouched by the slab.