Final answer:
The best operational definition of introversion is the frequency of initiating social activities, which aligns with introverts' preference for solitude and internal focus.
Step-by-step explanation:
The best operational definition of introversion is d) the number of times one initiates activities with friends on a weekly basis. This measurement reflects the tendency of introverts to seek less social interaction and more solitary activities, which corresponds with the energy they derive from their inner psychic activity as defined by Jung. Introversion is a personality trait characterized by a preference for quiet, less social stimulation, and more internally focused experiences.