a. is written in the middle voice. It is the same as "this book reads well/this pen writes smoothly/this car drives badly"
b. is written in the passive with an implied agent "by the manufacturers"
c. the active form would be "The manufacturers have improved their alarm clocks."
The only difference is that the first uses an intransitive verb to give a simple statement, whereas the second uses a passive verb to suggest that someone (the manufacturers) have made the improvements.
Your choice will depend on the context. If you are talking about mobile technology, use the first. If you are talking about manufacturers, use the second.
I'm not at all troubled by the absence of the passive. It's becoming more and more frequent and we are already familiar with similar constructions such as 'the plane flew very low', 'this reads like an advertisement', 'the cake cooked well'.
It is known as the 'middle voice' when the subject both performs and receives as both subject and agent.
It's rather a philosophical point. If you think that progress is more or less inevitable, like natural evolution, you will probably prefer the version: "Alarm clocks have improved to meet different people’s needs." (Note that I've deleted "The". We're talking about clocks in general over a period of time.)
But if you don't have in mind that generalised philosophy of long-term improvement, then "The alarm clocks have been improved to meet different people’s needs" suits your meaning better. You're being very explicit that this is the manufacturers' action.
Sometimes people with the philosophy of long-term improvement will use the verb evolve, which makes their intention very clear.