All of the incorrect answer options are muscles of the anterior compartment of the leg. Muscles of the anterior compartment are involved in dorsiflexion of the ankle and are all innervated by the deep fibular nerve.

The fibularis longus is a muscle of the lateral comaprtment of the leg. Muscles of the lateral compartment of the leg are innervated by the superficial fibular nerve.

The piriformis passes through the greater sciatic notch and, in some people, is pierced by the sciatic nerve. 

We are looking for a muscle in the anterior/medial compartment of the thigh. Based on this we can eliminate the tensor fasia latae and vastus lateralis muscles because they are not in this compartment. The vastus lateralis muscle is part of the quadriceps, meaning it inserts into the tibial tuberosity via the quadriceps tendon, and is the lateralmost of the four quadriceps muscles. The adductor brevis does originate on the pubis, but it inserts on the femur, not the tibia, and its action is adduction of the femur. That leaves us with only the gracilis muscle as the right answer. The action of the gracilis is primarily flexion of the tibia, but it also medially rotates and adducts the hip.

The semitendinosus, gracilis, and sartorius are the three muscles that make up the pes anserine group, coming from the posterior, medial, and anterior compartments of the thigh, respectively. The adductor longus, is not part of this group as it inserts on the linea aspera of the femur, rather than the pes anserinus, like the semitendinosus, gracilis, and sartorius.

No muscles attach to the talus directly. However, the extensor digitorum brevis comes close, via its attachment to one of the talocalcaneal ligaments. 

The achilles tendon or calcaneal tendon inserts on the posterior side of the calcaneus. It is the largest tendon in the body and pulls the heel up to point the toes downward. This decreases the angle between the bottom of the foot and back of the leg. This is called plantar flexion.

The flexor hallucis longus attaches to the plantar surface of the hallux or big toe. Much of the spring force (originating in the foot) when walking or running is generated by the contraction of this muscle. The popliteus and gastrocnemius muscles are in the crural region. The flexor digitorum longus is also in the foot, but acts to flex the second, third, fourth, and fifth toes which generate less push force.

The fibularis (peroneus) longus depresses the base of the first metatarsal, allowing the body to advance forward, over the limb that is in contact with the ground (the stance limb).

The soleus is the correct answer. The gastrocnemius also inserts on the calcaneus, but it originates on the condyles of the femur. Th plantaris muscle also inserts at the calcaneus, but it also originates at the femur. The biceps femoris muscles are located on the posterior side of the thigh. The sartorius originates on the anterior superior iliac spine, and inserts on the anteromedial aspect of the tibia.

The gastrocnemis is a knee flexor, so it would promote knee buckling, not resist it. Although the tibialis anterior does not cross the knee joint, keep in mind that in weight bearing, ankle dorsiflexion is coupled with knee flexion. Accordingly, the tibialis anterior would also promote knee flexion, instead of resisting it. By the same principle, the soleus prevents knee buckling/controls knee flexion by eccentrically controlling ankle dorsiflexion.