In Which of the Circumstances Outlined Below Is There an Omission

In which of the circumstances outlined below is there an omission to which liability might potentially attach?

A) A small boy falls into a lake and a passer-by jumps in to try and save him, but is unable to and the boy drowns.
B) A small boy falls into a lake and a passer by jumps in to try and save him, but inadvertently pulls the boy under the water. The boy drowns.
C) A man is walking along the road engrossed in his newspaper - he is about to step off the curb into the path of an oncoming bus. Another person sees this about to happen, but does nothing.
D) A father takes his child to the park. On the way there he notices the child about to run out on the road, into the path of an oncoming bus, but does nothing