Which example action corresponds to the extensor carpi radialis?

The extensor carpi radialis is a muscle located in the forearm that primarily functions to extend and abduct the wrist. The action referred to in the correct choice involves using this muscle to perform a movement where the wrist is extended against gravity.

In the case of a reverse wrist curl, the positioning involves holding a weight with the palms facing down and then curling it upwards, which specifically activates the extensors of the forearm, including the extensor carpi radialis. This movement focuses on extending the wrist, highlighting the muscle's role in lifting the back of the hand toward the forearm.

In contrast, the other actions listed—wrist curl, bicep curl, and cable push down—primarily engage muscles responsible for flexion rather than wrist extension. The wrist curl specifically targets the flexors, the bicep curl focuses on elbow flexion primarily involving the biceps brachii, and the cable push down emphasizes tricep involvement for elbow extension without engaging the wrist extensors in a significant manner. Thus, the reverse wrist curl is the only option that reflects the primary action of the extensor carpi radialis.