Overtraining typically occurs when an individual exercises too intensely or too frequently without allowing adequate recovery time for the body to heal and adapt. Among the options you provided, "exercising at a high intensity" and "exercising more often" can both lead to overtraining if not balanced with proper rest and recovery.
However, if we were to choose the one situation most synonymous with overtraining, it would likely be "exercising at a high intensity". While "exercising more often" could also lead to overtraining, high-intensity workouts without adequate recovery are a common cause of this condition.
Exercising within your comfort level and using repetitive motions by themselves do not necessarily lead to overtraining, though they can have other negative effects (such as boredom or increased injury risk).