Will the Earth ever stop rotating? It is well known that the Earth's rotation is slowing down and that millions of years ago there was a point in time where there was only a mere 20 hours in a day on Earth.

My question is in two parts.  Firstly why is the Earth's rotation slowing given that angular momentum is conserved, and secondly given that understanding, will the Earth either (a) stop rotating in the future, or (b) eventually begin rotating in the reverse direction to what it is now or will it (c) reach a constant rotational velocity in the same direction it is rotating now and why?

I think the answer to the question will depend on whether the Earth's rotation is slowing at a decreasing rate or at a constant rate.
The short answer is "Probably Yes".

The longer answer is that it is debatable whether Earth's rotation will become tidally locked to the moon due to tidal drag, at which point it will not be rotating relative to the moon, or whether the sun's exhaustion of hydrogen and the fusion of Helium will result in the sun's expansion vaporizing the earth first.  As tidal drag slows the earth's rotation, the moon moves further away, reducing its effect on the earth's rotation, and making the calculations fairly complex.

In either case, unless something really unexpected happens to the earth first, such as it being ejected from the solar system, which is completely improbable, it will stop rotating, if only due to it ceasing to be a planet at all.