What are these three called?

Fortress international, a large conglomerate, procures a few component parts from external suppliers and also manufactures some of the key raw materials in its own subsidiaries. aside from this, the company does not solely depend on outside distributors to reach its customers. in fact, it has its own retail stores to distribute its products. in this scenario, which of the following alternatives to vertical integration is fortress international applying? a. concentric integration b. taper integration c. horizontal integration d. conglomerate integration