The correct answer is: DRM permission is necessary when hand shunting outside station section takes place in gradient ST 1-260.
Hand shunting is the movement of a train by hand, without the use of a locomotive. It is typically used to move trains in and out of sidings, or to move trains around within a station yard.
Gradient is the slope of a railway track. It is measured in units of “per mille”, which is a percentage. A gradient of 1 in 260 means that the track rises 1 metre for every 260 metres of horizontal distance.
DRM permission is required for hand shunting outside station section in gradient ST 1-260 because it is a safety hazard. When a train is shunted by hand, it is not under the control of a locomotive. This means that it is more likely to derail, especially if the track is on a gradient.
The other options are incorrect because they do not represent the correct gradient. Option A, ST 1-600, is a gradient that is too steep for hand shunting. Option B, 1-400, is a gradient that is too shallow for hand shunting. Option D, None of these, is incorrect because it does not represent any of the possible gradients.