"C" with caution.
There is a big difference between 1 damsel and 1 clown in a tank vs. 6 damsels.
Plus, the damsels are already established. My fear is the clownfish will be harrassed to death.
As it is, 100 litres is probably too small for 6 damsels. My frag tank is 100 gallons and I have 2 damsels in it. Had them for years now. They fight on a fairly regular basis. I'm confident that if I tried to add another, he would end up as food for the hermit crabs!