In the resource$-$based model of above$-$average returns, a core competency is something that:

a$.$ a company is really good at internally and for which customers are willing to pay a premium.

b$.$ positions strategic leaders for success in managing people, processes, and resources.

c$.$ prevents a company from producing above$-$average returns.

d$.$ gives a company an edge over the competition, something that the company possesses that its competitors do not have, and something that is too difficult or costly for competitors to copy or produce an alternative.