Final answer:
Caffeine and citrus aurantium are considered stimulants, leading to the correct answer option b) II and IV. Creatine and HMB are not stimulants but assist in energy production and muscle recovery, respectively.
Step-by-step explanation:
Among the dietary supplements listed, caffeine and citrus aurantium are considered stimulants. Caffeine is widely known for its stimulant properties, providing increased levels of alertness and arousal. Its primary mechanism of action involves antagonizing adenosine activity, leading to stimulation. Citrus aurantium, also known as bitter orange, contains synephrine, a compound similar to ephedrine, which can also produce stimulant effects. On the other hand, creatine is an organic acid that supplies energy to muscle cells and has been shown to improve strength, energy, muscle mass, and recovery times. Creatine is not a stimulant but rather aids in energy production during high-intensity exercises. HMB, or beta-hydroxy beta-methylbutyric acid, is a compound that aids in muscle recovery and is also not classified as a stimulant.
Thus, the correct answer is option b) II and IV, which lists caffeine and citrus aurantium as the stimulant dietary supplements.