b) an "abstract" conviction that it is the right thing to do
c) fear of guilt if the action is left undone.
These work in combination, of course, and are to an extent mutually dependent, or at least connected. You wouldn't feel guilt if you didn't think kindness was right, for example. But in so far as it's possible to separate them out, the order in which I've listed them is the order in which (it seems to me) they are most valued in this society. For example, I would like to minimise the proportion of c) (which smacks of self-interest) while maximising a) and (to a lesser extent) b).
On the other hand, I imagine there are people for whom the top two places are reversed - the Kantians among us, for example. And it also occurs to me that there's an element of neurotypical bias in place in valorising empathy. And also self-interest again - for if c)'s fear of guilt looks to protect one's own psyche, let us not deny that a) comes with its own reward in the form of a little endorphin rush - so there's self-interest there, too.
By the time I had finished turning all this over in my mind, the beggar was half a mile down the street.