What does the "Talent Triangle" refer to$?$

a$.$ The project teams' talent in managing the three constraints of scope, time and cost.

b$.$ The interaction of the three executive$-$level roles on the project: Sponsor, Steering Team, and Project Management Office.

c$.$ PMI's research findings that project managers need skills and knowledge in three areas: Technical, Behavioral, and Strategic.

d$.$ The hierarchy of project management life cycles, knowledge areas and process groups.