I've worn earplugs while cycling for years; the protective type, as in (d) above. It cuts down on traffic noise and makes the ride a lot more pleasant. I don't feel it affects my ability to hear oncoming cars that much, and in any case, eyes and mirrors have the main job of doing that. My rides are always noisy, whether its from city traffic or the windblast of a good downhill run out in the country.

My ears are my friends. I've watched older relatives lose their hearing and it's not pretty. From what I gather, living with hearing aids is like having a tinny 1960s transistor radio in your head. And they cost as much (or more) than a high end road bike.