Thank you for all of the answers

[dylanfanatic]

Thank you folks. The stupid answer is b)speed obviously.
Regards other answers I tend to agree that driving through slowly is the best option. However, I once had to stop on the hard shoulder on the M60 in the rush hour, torrential downpour and my wipers had packed in. The question is ambiguous because you have to assume that the water is not deep. To those who say don't stop on a motorway, that is precisely what the hard shoulder is there for!