Which statement most accurately describes a randomized controlled study design?

Which statement most accurately describes a randomized controlled study design?
a.   The researcher examines the individual histories of a group of people manifesting a particular disease to find out what common factors they share and what differences can be discerned
b.   Questions and interventions that evolve through partnerships between researchers and participants
c.   This is the best design for answering questions about the effectiveness or harms of a certain intervention
d.   Questions exploring perceptions, feeling, and experiences are best answered by this study design