**Core Concept**
The hamstrings are a group of muscles in the posterior thigh responsible for knee flexion and hip extension. They are formed by three main muscles: biceps femoris, semitendinosus, and semimembranosus. These muscles work together to facilitate various movements and maintain posture.

**Why the Correct Answer is Right**
The correct answer includes semitendinosus (a) and semimembranosus (b). These two muscles are indeed part of the hamstring group. Semitendinosus is the most superficial of the two, and it plays a significant role in knee flexion, hip extension, and medial rotation of the tibia. Semimembranosus is deeper and also contributes to knee flexion, hip extension, and medial rotation of the tibia. Both muscles are innervated by the tibial nerve and have a common blood supply.

**Why Each Wrong Option is Incorrect**
**Option A:** Gracilis (c) is actually part of the adductor group, not the hamstrings. It is involved in hip adduction and knee flexion, but it is not a hamstring muscle.
**Option C:** Short head of biceps femoris (d) is a component of the hamstring group, but it is not the same as semitendinosus or semimembranosus. The short head is involved in knee flexion and hip extension, but it is a distinct muscle from the other two.
**Option D:** Sartorius (e) is actually a muscle of the anterior thigh, not the hamstrings. It is involved in hip flexion, knee flexion, and tibial rotation, but it is not part of the hamstring group.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the hamstrings are a group of muscles that work together to facilitate knee flexion and hip extension. Understanding their anatomy and function is crucial for diagnosing and managing conditions such as hamstring strains and tendinopathies.

**✓ Correct Answer: B. ab. Semitendinosus and semimembranosus are part of the hamstring group.**