How far above the maximum nut extension must the shank of a jack extend?

The correct answer is based on the safety standards and operational guidelines for scaffolding systems. The requirement for the shank of a jack to extend a certain distance above the maximum nut extension ensures stability and safety. A distance of 150mm is mandated because it allows sufficient space for load distribution and provides a safety margin to prevent the jacks from loosening under strain or shifting during use.

This measure also accounts for any slight adjustments that may be needed in the height of the scaffolding, ensuring that the structure remains secure and stable. If the extension were less than 150mm, there would be an increased risk of instability, as the jack may not fully engage with the structure it supports.

Other distances such as 100mm, 200mm, or 250mm could either compromise safety by not providing enough support or may exceed requirements unnecessarily, leading to impractical setups. Hence, the 150mm specification strikes a balance between safety and functionality, making it the correct answer.