"curtis and martin have been friends for seven years. after both people accepted jobs in different parts of the state, they called and e-mailed one another less until the relationship ultimately dwindled. which aspect of the relationship best explains the reason for the relationship's termination

Relationships require effort to succeed.

Explanation:

The reason for relationship termination is because Curtis and Martin took a job in different parts of the state therefore it seldom meets with each other.  

A relationship requires time and effort to flourish and succeed. But in this case, they are busy in their jobs and thus communicate less. Lack of communication and lack of time spent with each other deteriorate their friendship.