Final answer:
The term is 'authoritativeness'. It encapsulates the idea of being in control of a situation by expressing the necessary confidence and expertise to command respect and to guide others effectively.
Step-by-step explanation:
The term that can be used to describe the noun solidity emphasising the importance of Edwin being in control of the situation is 'authoritativeness'. This reflects the sentiment shared in the statement, 'The consummate leader cultivates the moral law, and strictly adheres to method and discipline; thus it is in his power to control success'.
The emphasis here is on the leader's ability to guide, direct, and ensure steady progress towards goals, which is a byproduct of exhibiting control and authority.
Being authoritative means possessing the confidence and expertise to command respect and, in turn, guide the decision-making process effectively. This trait is a keystone for individuals like Edwin, who are involved in leadership roles, ensuring that they maintain the course and integrity of the group or situation they are in charge of.