All the options presented can be relevant to the role of a personal trainer, but the most characteristic traits of personal trainers would be:
C. Correcting exercise form
D. Motivating clients
While designing injury rehabilitation (A) is more aligned with physical therapists and scouting athletic ability (B) is more common in sports coaches, C and D focus on the fundamental aspects of personal training, which include ensuring clients perform exercises safely and effectively and providing motivation to help them achieve their fitness goals.
So, the best answers are C and D.