A rustler is best defined as someone who steals what?

A rustler is primarily defined as someone who steals livestock, particularly cattle or horses. This term has its origins in the American West, where cattle ranching was a significant aspect of the economy. The act of rustling was often associated with the lawlessness of the frontier, where stealing livestock was a common crime due to the high value placed on animals for ranching and trade. Cattle rustling specifically became a major issue in places like New Mexico, where ranchers needed to protect their herds from thieves.

The other options, while related to criminal activity, do not fit the specific definition of a rustler. Guns and money thefts are categorized under other forms of crime, and land theft typically involves different legal disputes and conflicts, especially given its significance in property rights. Rustling, however, directly points to the stealing of animals, making "livestock" the accurate choice in this context.