(c) No, you did not choose the best answer. It is not correct to say that a legal obligation arises to avoid harm only if the harm is more likely than not to happen. The rule is that an obligation to avoid harm arises if there is 'a real risk' that the harm will occur - a risk that is not so 'negligible or so remote that a reasonable person would reject it as unworthy of consideration'.
From this it can be seen that it is not necessary to show that the harm was more likely to happen than not.
In the present case, it seems that the risk of an accident is a real risk. The possibility of an accident is not far-fetched or fanciful: it is something that is reasonably likely to happen. The responsible authority would therefore be required to take steps to prevent it.