What are the sources of energy for a Space Transit Habitat(manned) in terms of both propulsion( after passing over the Low Earth Orbit) and required electric facilities)through a 6-8 month voyage to Mars?
Your options are pretty much chemical, solar and nuclear. Solar isn't as bad at Mars as farther out, as it is only half at Mars what it is on Earth. However, you have the problem of having to point a large flat structure at the sun, the thrust in a second direction and the communication dish in a third direction. Not very elegant. Chemical is doable for only 6-8 months but would be a bit massive - on the order of a few hundred tons. You still have the problem of what to do when you land.
Nuclear is the future for space travel as it is over a million times as energy dense as chemical. A steel can the size of kitchen trash can might be enough do the trick. The only problem is that anti-nuclear sentiment might delay your departure until the year 9999.
If you go nuclear, your options are nuclear thermal and nuclear electric. For nuclear thermal, the thruster was fully developed by 1971 (NERVA) but no longer exists. The onboard electric would be generated by a 'bimodal' approach in which some of the nuclear heat also drives a generator. It's advantage would be that it could 'refuel' on any fluid (gas or liquid) available in space or transferable from a destination (water, hydrogen, methane, CO2, etc). However, you would have to build expensive scrubbing facilities in which to test it during (re)development.
Nuclear electric is easier in the sense that you just have to use a small nuclear reactor of a type similar to many already tested at national labs and universities. Electric thrusters of at least a half dozen architectures have been lifetime tested and shown usable. The overall weight of the reactor is much greater for the energy output as compared to nuclear thermal, but exhaust velocities of electric thrusters are much higher, offsetting this. There is also a better chance of having a president or black ops project push it through quickly and fly it secretly before the lobbyists can whine about it.