To help Dax determine if starting a business with his friend meets his needs, the key question he should focus on among the options provided is:
A. How much control of the business do you want?
This question directly addresses his preferences for control and decision-making authority, which are critical considerations in any partnership arrangement. Understanding how much control he wants will help him evaluate the dynamics of the partnership with his friend, especially given his inclination to prefer being in charge.
While the other options may also be relevant to starting a business, they are not as directly related to his need for control and partnership dynamics.