Exercising control over (A).
But you can't say "houses prices". You could say "house prices" or, more natural, "housing prices".
"Exercising control over housing prices is a matter that most people have been worrying about."
"...that most people have been worrying about" is an adjective clause (also called "relative clause"). It describes the noun "matter". One of the things people worry about is (the government? the property owner's?) controlling the prices of houses.
With "exercising" control, the idea is that the government or property owner's or real estate agents, have the right to control the prices.