Final answer:
The best characteristic to achieve fitness goals is self-control, essential for maintaining behaviors necessary for reaching targets and fostering a congruent self-image.
Step-by-step explanation:
The characteristic that would best help a student who is focused on achieving fitness goals is d. Self-control. Self-control, also known as self-regulation or will power, is the ability to control impulses and reactions, and is essential for maintaining consistent habits and behaviors necessary to reach fitness targets. The concept is closely related to self-efficacy, which is one's confidence in their own abilities, and is crucial for setting and reaching personal goals. Developing self-control can also lead to a higher level of congruence, where one's actions align with their self-concept, further supporting progress towards fitness goals.