Earth Fault in a TT Network: Current loop, or charges flowing from one grounding to the other?
It's essentially option (b).  The fault current travels through the Earth between the two grounding rods.

Parts of the Earth may have a high resistivity, but at the same time, it is a really fat conductor - about 12000 km at its widest!

But, the resistances where the earth rods meet the ground, modelled as RA and RB, may be quite substantial.  This means that the fault current can be quite low, often not even enough to blow a fuse or trip a breaker.  That is why a TT installation should have an RCD (or GFCI).