39. An employer shall ensure that a fork-lift truck carrying one or more employees is equipped or adapted to limit the risk of it overturning—

(a) by the installation of an enclosure for the driver,

(b) by a structure preventing the fork-lift truck from overturning,

(c) by a structure ensuring that, if the fork-lift truck overturns, sufficient clearance remains between the ground and appropriate parts of the fork-lift truck for the F11[ride-on employees], or

(d) by a structure restraining the employees on the driving seat so as to prevent them from being crushed by parts of the fork-lift truck which overturns.