You can try Cat6 cable. That's the best grade of ethernet cable. Ethernet is spec'ed to work with cables as long as 100 meters, so it should work, maybe even further. But almost nobody uses cables that long so it's hard to predict if it will actually work. If the router/switches are up to snuff, then you should be able to hit 100 meters. If they're not, then you may not even get past 80 meters.
You're probably not going to be able to find pre-made 100 meter cables, so you'd need to make it yourself. A 1000' bulk spool of Cat 6 is about $130-$150. And the cutting and crimping tools are about $20-$50. The ethernet jacks themselves are about 10-20 cents each depending on quantity. If you really want to go this route, ask here and we can instruct you on how to make the cables.
Given how much it would cost to make your own cables, you may be better off going wireless. If you have line of sight to both houses (no other houses in between), I've had good luck with Ubiquiti's Nanostation M units. They're directional wifi antennas. The 2.4 GHz model is about $40, the 5 GHz about $55. Try one at B and C and have them connect to your router at A. They should be able to connect at that distance even without your router having directional antennas. (They're spec'd to work out to 5-10 km with directional antennas at both ends.) If it doesn't work, you'll have to add another Nanostation at A. If it comes to that, hopefully B and C are in roughly the same direction so you can get away with just a single Nanostation at A..