During bromination using NBS, the addition product is not observed because the rate of addition is than the rate of the chain propagation steps.

slower

Explanation:

NBS is used in place of Br2 during bromination because the use of Br2 tends to result in the formation of a large concentration of the addition product.  This is because, the Br2 interacts with the double bond to yield only the dibromide (addition product).

The usefulness of NBS lies in the fact that it provides a lower concentration of Br2, and such that the rate addition is slower than the chain propagation steps. Only the monobromination product is observed

The low concentration of Br2 allows the free-radical reaction to out-compete the alkene addition reaction.

im not exactly sure but i think its b!