The correct answer is C) spring force. In a compression type automatic grease cup, the lubricant is forced into the bearing by the spring force. The spring inside the grease cup applies constant pressure, which pushes the lubricant out through the outlet and into the bearing. This steady, controlled flow of grease ensures proper lubrication without the need for manual intervention. The other options are incorrect because: A) a pressure gun is used to manually force grease into a bearing, not an automatic system; B) a Zerk fitting is simply the grease fitting that allows access to the bearing, not the mechanism that forces the grease in; and D) gravity flow alone is not sufficient to consistently deliver the required amount of grease to the bearing in an automatic system.