Place any small object inside a cavity whose walls are perfectly black and held at a uniform temperature T. Because the walls are black, they flood the cavity with isotropic blackbody radiation of spectral intensity I_b,lambda(T), independent of what is placed inside.
Let the small object have spectral absorptivity alpha_lambda and spectral emissivity epsilon_lambda. The power it absorbs per unit wavelength interval is alpha_lambda I_b,lambda(T), and the power it emits per unit wavelength interval is epsilon_lambda I_b,lambda(T).
Once the object reaches thermal equilibrium with the enclosure, its temperature stops changing, which means it cannot be a net emitter or a net absorber at any wavelength band. So, wavelength by wavelength: epsilon_lambda = alpha_lambda.
Check the four options against this result: reflectivity is unrelated in general; the Stefan-Boltzmann constant is a universal constant unrelated to any specific surface; zero would mean the body neither absorbs nor emits at all, contradicting it reaching thermal equilibrium by radiative exchange.
\[ \boxed{\text{Answer: absorptivity of the body at the same wavelength}} \]