The driver of the car in the centre of the picture has pulled over to the side of the road to answer their phone. Why is it prohibited here?
Review of the options:
The weather means that this may present a traffic hazard – It is certainly raining, but visibility still seems to be good and it is not dark. The weather is therefore not a big enough problem for stopping to be classed as a traffic hazard.
The centre line is solid, which prevents the traffic behind from overtaking – It looks like there is a margin of at least two car widths to overtake on, so there is no problem with the margins. The fact that the line is solid does not automatically entail that overtaking is prohibited.
The road is a priority road – It can be observed that the road is a priority road by looking at the back of the sign under the 70 sign. The sign below the 70 sign could signal the end of the priority road, but by applying the exclusion method to the other options, you can assume that it is not an end sign (no option would be correct if it was an end sign).
Answering a mobile phone counts as parking, not stopping. See the Driving Licence Book (19th Edition, page 104): “The parking prohibition on the priority road means that you may not even stop and answer your phone”.
As parking is prohibited on priority roads (assume that it is a priority road, see the above point), it is prohibited to answer a mobile phone at this location.