Understand the concept of the equilibrium constant (K_c) and its relation to reactant and product concentrations.

The equilibrium constant ($K_c$) indicates the extent to which a reaction proceeds to completion.

• If $K_c$ is very large ($K_c \gg 1$), the equilibrium lies far to the right, meaning products are heavily favored, and the concentration of reactants at equilibrium will be very small.
• If $K_c$ is very small ($K_c \ll 1$), the equilibrium lies far to the left, meaning reactants are heavily favored, and the concentration of products at equilibrium will be very small.
• If $K_c$ is neither very large nor very small, then both reactants and products will be present in appreciable (significant and measurable) concentrations at equilibrium.

In general chemistry, a common rule of thumb is that if the equilibrium constant $K_c$ is between $10^{-3}$ and $10^3$, then both reactants and products will be present in appreciable concentrations at equilibrium. Outside this range, one set of species (either reactants or products) will be present in very low concentrations.

The options are:

a) $10^{-4}$ to $10^4$

b) $10^{-3}$ to $10^3$

c) $10^3$ to $10^6$

d) $10^{-5}$ to $10^3$

Based on the established chemical principle, the range $10^{-3}$ to $10^3$ corresponds to the condition where appreciable concentrations of both reactants and products are present.

The final answer is $\boxed{\text{b) } 10^{-3} \text{ to } 10^3}$.