Final answer:
Option A, which involves 3 sets of 15 reps with light weights, would be the best choice for a female client who wants to improve strength without 'bulking up', as it focuses on muscle endurance and toning rather than hypertrophy.
Step-by-step explanation:
The question is focused on finding an appropriate weight-lifting set-repetition scheme for a female client who wishes to gain strength without 'bulking up'. Given the options provided, Option A: 3 sets of 15 reps with light weights is typically recommended for endurance training and muscle toning without significant hypertrophy (an increase in muscle size). This scheme promotes improved muscle endurance while minimizing the risk of developing large muscle mass.
High repetitions with lighter weights are less likely to produce the bulky muscle mass that your client is concerned about, as they do not provide the same level of intensity required for substantial hypertrophy that lower repetitions with heavier weights would. In contrast, Option B: 5 sets of 5 reps with heavy weights is usually aligned with strength building and could potentially lead to more muscle bulk, which is not the client's goal. Option C: 4 sets of 10 reps with moderate weights can be a compromise between strength and endurance training, but it may still lead to some level of hypertrophy. Lastly, Option D: 2 sets of 20 reps with light weights is also focused on endurance; however, it might not provide enough stimulus for strength gains compared to Option A.