As is well known, option C is very painful for the receiving partner, and impossible in the course of ordinary life.
But specifically during pregnancy it is fine.
It seems to me that this is how the Creator made nature.

But of course you have to ask permission first, and be careful that it not hurt.
If it hurts, stop.

In your opinion, is this permitted only in the above circumstances, or also in general, when she is not pregnant?

What emerges from the sources is that intercourse should be with the husband on top. At least regarding some of the points that are based on health considerations, one can discuss whether they are binding, and even with considerations of modesty there may be changes over time. But this is not the place to elaborate.