The best sampling method used for drug abusers: (A) Quota sampling, (B) Snowball sampling, (C) Cluster random sampling, (D) None of the above
Answer is B. Snow-ball Sampling

A technique for developing a research sample where existing study subjects recruit future subjects from among their acquaintances; thus the sample group appears to grow like a rolling snowball
Is often used in hidden populations which are difficult for researchers to access, e.g. drug users or commercial sex workers.