Final answer:
The barriers to entry in the tanzanite market are due to exclusive ownership of the mines by the Tanzanian government, which controls access to the essential resource, creating a monopoly. Option a is the answer.
Step-by-step explanation:
In the case of the tanzanite market in Tanzania, the barriers to entry that exist are primarily due to the exclusive ownership of a key resource, namely the tanzanite mines. The local government owns these mines and requires permission for anyone to access them, effectively preventing any potential competitors from entering the market. This situation does not arise from economies of scale or government-created monopolies in the usual sense, but from the total control over the access to the necessary raw material to produce tanzanite, which creates a monopoly scenario.