What causes the bottom string to break in the string-pull illustration?

The bottom string in the string-pull illustration breaks primarily due to the mass of the ball. When a force is applied to the string to pull the ball, the weight exerted by the ball, which is a result of its mass and the acceleration due to gravity, creates tension in the strings. The bottom string experiences tension that can exceed its breaking point depending on how much force is applied and the ball's mass.

When the ball is pulled, the gravitational force acting on it (which is proportional to its mass) determines the amount of tension generated in the string strands. If the tension from the mass of the ball exceeds the tensile strength of the bottom string, it will break.

While the weight is a factor, focusing on mass offers a clearer understanding, as it directly relates to the force due to gravity acting on the ball. The other choices, such as the speed or angle of the pull, while they can influence how dynamic forces are applied, are secondary in this context to the fundamental effect of the mass creating the initial tension in the strings.