Final answer:
Quotation d) is the correct choice as it most directly demonstrates the theme that goals can be achieved by persistently increasing effort and improvement in performance.
Step-by-step explanation:
In evaluating the provided quotations to determine which suggests that the theme is that goals can be achieved through persistence and preparation, it is important to identify elements that emphasize effort, planning, and consistent action towards achieving an aim.
Quote d) "She ran at least three times a week—sometimes more—and kept increasing her distance and improving her pace." most directly reflects the notion of persistent training and preparation towards a goal, in this case, improving performance in running.
The actions described in the quotation imply a disciplined and regular effort, reinforcing the theme that consistent hard work and preparation can lead to the achievement of goals.