Which of the following techniques refers to a 'hammer fist'?

The technique that is referred to as a 'hammer fist' is known as Yuk soo. The hammer fist involves striking an opponent with the side or bottom of the fist while the arm is in a downward or horizontal position. This method can generate significant power due to the body mechanics involved and the ample surface area of the fist, making it effective in self-defense situations.

In contrast, Soo do typically refers to a knife-hand strike, which utilizes the edge of the hand, while Jireugi means 'punch' and usually focuses on the force of a direct fist impact. Sonnal also refers to a knife-hand technique that emphasizes precision and targeting with the side of the hand. Yuk soo stands out in this context as the technique specifically describing the action of delivering a strike similar to that of a hammer, thus correctly associating it with the term 'hammer fist.'