Final answer:
Anabolic steroids are the most likely substance to increase lean body mass among the options given.
Step-by-step explanation:
The performance-enhancing substance most likely to increase lean body mass among the choices provided is A) anabolic steroids. Anabolic steroids are synthetic variations of the male sex hormone testosterone and are designed to simulate the muscle-building effects of this hormone. They have been proven effective in increasing muscle mass and are commonly used for this purpose despite the associated health risks and legal issues. Other substances listed, like arginine and beta-alanine, may have various impacts on performance but are not as directly associated with muscle mass increase as anabolic steroids. Ephedrine is a stimulant and not directly involved in muscle building.