Concept: A bureau-de-exchange is a specialized business that deals mainly in the exchange of foreign currencies.
Explanation:A bureau-de-exchange buys foreign currency from individuals and businesses at one rate.
It sells foreign currency to customers at a slightly higher rate.
The difference between the buying rate and the selling rate is its profit margin.
Therefore, its basic function is to convert one currency into another.
Option A is incorrect because providing trade information is not the central role of this business.
Option C is incorrect because buying and selling securities is the function of the stock market.
Option D is incorrect because selling goods and services belongs to ordinary traders.
So the main function of a bureau-de-exchange is exactly the activity described in option B.
Answer: B. buying and selling foreign currency