d
Hollow enclosure blackened inside and having a small hole is a very good example of a black body.

Suppose once light enters inside it.

It may be absorbed or it may be reflected.

Since it is blackened from inside, there is a high probability that it will be absorbed.

Now if it is reflected, it will suffer multiple reflections and it is very unlikely that it will come out of the hole because the aperture of hole is too small. Moreover, with each reflection, more and more fraction of it will be absorbed. So, it will serve as a good black body.