Most grocery stores carry a wide variety of national brands in addition to their own private$-$label brands. The private$-$label brands are often made by the same national brands they are competing with. Which type of strategy is a firm using when it creates a lower$-$priced private$-$label brand in addition to its other, higher$-$priced brands?

a$.$ Distinctive competence

b$.$ Differentiation

c$.$ Portfolio management

d$.$ Cost leadership

e$.$ Focus